Support creating specialized vector/matrix functions

[miselin]

fvecN and matMxN both exist as statically defined types when a programmer writes their Haven code. This is fine, but it made it difficult to imagine building custom libraries for future development work.

Consider something like ML code, where we might want to have tensors of varying lengths. The current approach requires the library implementor to either pre-specialize all the possible shapes or have some sort of code generation. Not great!

This PR instead adds specialization hole capability, where fvec? and mat? can be provided as parameters. These are specialized at compile time. There are a few compile-time constants (.dim on vectors, .cols/.rows on matrices) that present the "real" shape once automatically specialized. A compile-time assertion in the form of @assert now also gives implementors the ability to validate the input parameters statically.

Also, functions using specialization holes will infer their return types automatically.

⚠️ CI will no longer run the legacy C compiler after this change! This is a real syntax change that won't be backported to the C version, marking this the first backwards-incompatible change.