Adds internal/stats package

[gbotrel]

Previously, we did check for regression of internal/circuits (number of constraints and number of wires).

But not for std/ circuits.

This PR closes #212 and adds internal/stats package.

Enables registering "snippets", for example (see internal/stats/snippet.go):

registerSnippet("math/bits.ToBinary", func(api frontend.API, v frontend.Variable) {
    _ = bits.ToBinary(api, v)
})

will measure statistics for the provided snippet.

To regenerate statistics, in internal/stats/generate , run go run main.go -s.

Running go run main.go -v will generate a csv output. For example, running go run main.go -v -run="ToBinary$"

will output:

id,curve,backend,nbConstraints,nbWires
math/bits.ToBinary,bn254,groth16,255,254
math/bits.ToBinary,bn254,plonk,508,507
math/bits.ToBinary,bls12_377,groth16,254,253
math/bits.ToBinary,bls12_377,plonk,506,505
math/bits.ToBinary,bls12_381,groth16,256,255
math/bits.ToBinary,bls12_381,plonk,510,509
math/bits.ToBinary,bw6_761,groth16,378,377
math/bits.ToBinary,bw6_761,plonk,754,753
math/bits.ToBinary,bls24_315,groth16,254,253
math/bits.ToBinary,bls24_315,plonk,506,505

Remaining:

note: Not urgent, but upcoming PR may have a "post compile" phase that reduce un-referenced constraints, which might make snippets stats less elegant.