feat: add BabyBear and KoalaBear 31bits fields vanilla Go impl

[gbotrel]

Description

Adds 31bit fields support (i.e. template generate Golang code with uint32).

Perf work and vectorization (AVX, NEON) to come in future PRs.

		{"koalabear", "0x7f000001"}, // 2^31 - 2^24 + 1 ==> the cube map (x -> x^3) is an automorphism of the multiplicative group
		{"babybear", "0x78000001"},  // 2^31 - 2^27 + 1 ==> 2-adicity 27

Unit benchmarks are misleading; while it does seem that koalabear.Mul is slightly slower than goldilocks.Mul , the code generated by the compiler is as compact as it could be (with no branch) and is 50 to 70% more performant when inlined in vector ops for example.