Refactor arithmetic with macros + generics

[jdwhite48]

There is a significant amount of code re-use in data_structures.rs, which makes maintaining the code unreasonably difficult to work with; in particular, Com1 and Com2 are implemented incredibly similarly.

• Figure out if it's possible for a macro to include a "match" statement on the identifier passed as input such that, if it equals a certain value (e.g. Com1), one can define a new identifier macro variable based on it (e.g. G1Affine)
• Coalesce Com1 / Com2 implementations to a greater extent (and also ComT, if it shares any similarities that can be macro'ed out)
• Similarly, coalesce Mat implementations for Com1, Com2, and/or field matrices, if possible (may require implementing Com1 and Com2 as a Field, or specifying the current Field generics at a finer-grained level)
• Optionally, consider defining a mat![ a, b, c; ... ; ...] macro (or similar) which expands out to vec![vec![a, b, c], vec![...]. vec![...]

This should be transparent to the API and test cases.