adds sub-project with benchmarks

[heckj]

• Benchmarks replicated (but not identical) to the code in NoiseTests - this doesn't remove that code
• Uses package-benchmark, with instructions for how to run/compare against a baseline also stored within that subproject
• Set up in a subdirectory as a project with a local package reference to the root of this repository in order not to spill any dependencies to downstream consumers.
• I don't have a "proper" baseline setup, but I created one anyway with my M1 macbook pro, running only the benchmark in a terminal window, to get close.
• baseline added is bdb4ef08 - which was the commit before I rebased and compressed some history (doh!), but I left it rather than changing it. This is effectively a benchmark on my laptop from the 1.0.0 tag codebase with no functional changes, only updates to dependencies and swift compiler versions.

Goal of all this is to be able to quick check future changes against the pre-existing implementation, especially with an eye to enabling SIMD, evaluating the inline references, etc.