Add a dasp_graph crate.

[mitchmindtree]

This adds a new crate for working with dynamic audio graphs.

From the new docs:

dasp_graph is targeted towards users who require an efficient yet
flexible and dynamically configurable audio graph. Use cases might
include virtual mixers, digital audio workstations, game audio systems,
virtual modular synthesizers and related usecases.

This work has been a long-time coming and is the result of many
discussions in the rust audio community and many lessons learned over
the last few years of working with rust and audio. In particular:

• the development of and reflection on dsp-chain and its shortcomings.
• The (reasonable) limitations of dasp_signal when dynamically configuring graphs.
• Discussion on the design of audio graphs with @raphlinus at Lock-free processing dsp-chain#141.
• The development of the spatial audio server.
• A recent email discussion with Sami Perttu on DASP and audio graphs.

dasp_graph is of course not a one-size-fits-all solution. Instead, it
is designed specifically to work well alongside (and fill a gap within)
the rest of the dasp crate ecosystem. Please refer to the "Comparing
dasp_signal" section of the dasp_graph root documentation for a more
detailed overview of the design choices between the two, what
applications each are best suited toward and how the two best
interoperate together.

A small suite of node implementations are provided out of the box
including a Delay, Sum, Pass, GraphNode and BoxedNode, all of
which can be enabled/disabled via their associated features.

Following this, I have some ideas for adding an optional sync module
to the crate, aimed at controlling and monitoring a dasp graph and it's
nodes from a separate thread (i.e. for convenient use alongside a GUI)
in a non-dynamically-allocating, non-blocking manner. The work so far
has been performed with these plans in mind. The ideas are for the most
part based on the discussion at RustAudio/dsp-chain#141.

Also, no_std support for dasp_graph is currently blocked on petgraph
support for no_std. A PR is open for adding no_std support at
petgraph/petgraph#238. In the meantime, the std feature must be
enabled to use the new dasp::graph module. This is also noted in the
updated docs.

For more information about the crate and inner workings feel free to
read through the new dasp_graph docs. I'm yet to add examples, but
hopefully the added tests can give a good idea of how to use the crate
in the meantime.