Make crate no_std compatible

[olanod]

Make crate no_std while keeping it backwards compatible with a default std feature enabled(only used to impl Error for ParseError).

[LastLeaf]

It seems that the changes are not complete. I still find some deps need changes or default-features = false .

[olanod]

I tested it with cargo build --no-default-features and cargo test with default features on purpose to use std. Is that what you tried?

[LastLeaf]

I tested it with cargo build --no-default-features and cargo test with default features on purpose to use std. Is that what you tried?

I built with --target armv7a-none-eabi (which does not have a std) and fixed some of the compilation errors. You can take a look at my branch. It also requires a dep change.

[olanod]

Merged your branch as your PR for the dependency was merged and released.

[LastLeaf]

More changes are needed in the cssparser-color crate. My branch has been updated.

[emilio]

looks ok, but can you rebase and add the license headers? Thanks