tests: fix test build on musl

Using source checkout in core/lib/libfuzzer-sys with updated
llvm libfuzzer directory to todays release/13.x branch
which fixes a missing `#include <cstddef>`.

See rust-fuzz/libfuzzer#87

core/lib/fuzz/Cargo.toml

@@ -10,7 +10,7 @@ edition = "2018"
 cargo-fuzz = true
 
 [dependencies]
-libfuzzer-sys = "0.4"
+libfuzzer-sys = { version = "0.4", path = "../libfuzzer-sys" }
 
 [dependencies.rocket]
 path = ".."

core/lib/libfuzzer-sys/.gitignore

@@ -0,0 +1,2 @@
+target
+Cargo.lock

core/lib/libfuzzer-sys/CHANGELOG.md

@@ -0,0 +1,170 @@
+## Unreleased
+
+Released YYYY-MM-DD.
+
+### Added
+
+* TODO (or remove section if none)
+
+### Changed
+
+* TODO (or remove section if none)
+
+### Deprecated
+
+* TODO (or remove section if none)
+
+### Removed
+
+* TODO (or remove section if none)
+
+### Fixed
+
+* TODO (or remove section if none)
+
+### Security
+
+* TODO (or remove section if none)
+
+--------------------------------------------------------------------------------
+
+## 0.4.2
+
+Released 2020-05-26.
+
+### Changed
+
+* Improved performance of checking for whether `cargo fuzz` is requesting the
+  `std::fmt::Debug` output of an input or not. This is always false during
+  regular fuzzing, so making this check faster should give slightly better
+  fuzzing throughput.
+
+--------------------------------------------------------------------------------
+
+## 0.4.1
+
+Released 2020-05-13.
+
+### Added
+
+* Added support for defining custom mutators. See [the documentation for the
+  `fuzz_mutator!`
+  macro](https://docs.rs/libfuzzer-sys/0.4.1/libfuzzer_sys/macro.fuzz_mutator.html)
+  for details.
+
+### Changed
+
+* Upgraded libfuzzer to llvm/llvm-project's 70cbc6d.
+
+--------------------------------------------------------------------------------
+
+## 0.4.0
+
+Released 2021-02-24.
+
+### Changed
+
+* The public `arbitrary` dependency was updated to version 1.0.
+
+--------------------------------------------------------------------------------
+
+## 0.3.5
+
+Released 2020-11-18.
+
+### Changed
+
+* [Upgrade libfuzzer to 7bf89c2](https://github.com/rust-fuzz/libfuzzer/pull/68)
+
+--------------------------------------------------------------------------------
+
+## 0.3.4
+
+Released 2020-08-22.
+
+### Changed
+
+* Updated `arbitrary` dependency to 0.4.6
+
+--------------------------------------------------------------------------------
+
+## 0.3.3
+
+Released 2020-07-27.
+
+### Changed
+
+* Upgraded libfuzzer to commit
+  [4a4cafa](https://github.com/llvm/llvm-project/commit/4a4cafabc9067fced5890a245b03ef5897ad988b).
+
+  Notably, this pulls in [the new Entropic engine for
+  libFuzzer](https://mboehme.github.io/paper/FSE20.Entropy.pdf), which should
+  boost fuzzing efficiency when enabled. You can enable Entropic by passing
+  `-entropic=1` to your built fuzz targets (although, note that it is still
+  labeled "experimental").
+
+--------------------------------------------------------------------------------
+
+## 0.3.2
+
+Released 2020-03-18.
+
+### Changed
+
+* Upgraded the `arbitrary` dependency re-export to version 0.4.1.
+
+--------------------------------------------------------------------------------
+
+## 0.3.1
+
+Released 2020-02-27.
+
+### Changed
+
+* Fixed a fuzzing performance issue where libfuzzer could unnecessarily spend
+  time exploring all the ways that an `Arbitrary` implementation could fail to
+  construct an instance of itself because the fuzzer provided too few bytes. See
+  https://github.com/rust-fuzz/libfuzzer/issues/59 for details.
+
+--------------------------------------------------------------------------------
+
+## 0.3.0
+
+Released 2019-01-22.
+
+### Changed
+
+* Now works with and re-exports `arbitrary` versions 0.4.x.
+
+--------------------------------------------------------------------------------
+
+## 0.2.1
+
+Released 2019-01-16.
+
+### Added
+
+* Added support for the `CUSTOM_LIBFUZZER_STD_CXX=<lib>` environment variable
+  during builds that already use a custom libFuzzer checkout with
+  `CUSTOM_LIBFUZZER_PATH`. This allows you to explicitly choose to link LLVM or
+  GNU C++ standard libraries.
+
+--------------------------------------------------------------------------------
+
+## 0.2.0
+
+Released 2020-01-14.
+
+### Changed
+
+* Using `arbitrary` 0.3.x now. It is re-exported as `libfuzzer_sys::arbitrary`.
+
+### Added
+
+* You can enable support for `#[derive(Arbitrary)]` with the
+  `"arbitrary-derive"` cargo feature. This is a synonym for the `arbitrary`
+  crate's `"derive"` cargo feature.
+
+--------------------------------------------------------------------------------
+
+## 0.1.0

core/lib/libfuzzer-sys/Cargo.toml

@@ -0,0 +1,30 @@
+[package]
+authors = ["The rust-fuzz Project Developers"]
+description = "A wrapper around LLVM's libFuzzer runtime."
+edition = "2018"
+license = "MIT/Apache-2.0/NCSA"
+name = "libfuzzer-sys"
+readme = "./README.md"
+repository = "https://github.com/rust-fuzz/libfuzzer"
+version = "0.4.2"
+
+[dependencies]
+arbitrary = "1"
+once_cell = "1"
+
+[build-dependencies]
+cc = "1.0"
+
+[features]
+arbitrary-derive = ["arbitrary/derive"]
+
+[workspace]
+members = [
+  "./example",
+  "./example_arbitrary",
+  "./example_mutator",
+]
+
+[dev-dependencies]
+flate2 = "1.0.20"
+rand = "0.8.3"