add benchmark for recovering cells without computing proofs

[cskiraly]

We will often need to recover cells only, without recovering the associated proofs, since proofs will already be available from a side-channel (e.g. as part of a message sent before the per-cell diffusion starts).

The API already supports this by setting the proofs array to NULL. This PR adds benchmarks.

[cskiraly]

Some reference numbers with the current branch on a Macbook Pro M1

goos: darwin
goarch: arm64
pkg: github.com/ethereum/c-kzg-4844/v2/bindings/go
cpu: Apple M1
Benchmark/ComputeCells               458           2799416 ns/op
Benchmark/ComputeCellsAndKZGProofs(precompute=0)                       4         280863979 ns/op
Benchmark/ComputeCellsAndKZGProofs(precompute=1)                       2         962029125 ns/op
Benchmark/ComputeCellsAndKZGProofs(precompute=2)                       2         519934146 ns/op
Benchmark/ComputeCellsAndKZGProofs(precompute=3)                       3         507837917 ns/op
Benchmark/ComputeCellsAndKZGProofs(precompute=4)                       4         293343365 ns/op
Benchmark/ComputeCellsAndKZGProofs(precompute=5)                       4         255128115 ns/op
Benchmark/ComputeCellsAndKZGProofs(precompute=6)                       5         226165750 ns/op
Benchmark/ComputeCellsAndKZGProofs(precompute=7)                       5         207891258 ns/op
Benchmark/ComputeCellsAndKZGProofs(precompute=8)                       6         189214535 ns/op
Benchmark/ComputeCellsAndKZGProofsParallel(count=8)                    4         328514510 ns/op
Benchmark/RecoverCells(missing=50.0%)                 55          22094399 ns/op
Benchmark/RecoverCells(missing=25.0%)                 54          21380794 ns/op
Benchmark/RecoverCells(missing=12.5%)                 55          29083810 ns/op
Benchmark/RecoverCells(missing=1)                     52          21519530 ns/op
Benchmark/RecoverCells(missing=2)                     54          21420107 ns/op
Benchmark/RecoverCellsAndKZGProofs(missing=50.0%)                      5         207627083 ns/op
Benchmark/RecoverCellsAndKZGProofs(missing=25.0%)                      5         208015733 ns/op
Benchmark/RecoverCellsAndKZGProofs(missing=12.5%)                      5         209756142 ns/op
Benchmark/RecoverCellsAndKZGProofs(missing=1)                          5         207826692 ns/op
Benchmark/RecoverCellsAndKZGProofs(missing=2)                          5         209587925 ns/op
Benchmark/RecoverCellsAndKZGProofs(missing=3)                          5         211216258 ns/op
Benchmark/RecoverCellsAndKZGProofs(missing=4)                          5         211892058 ns/op
Benchmark/RecoverCellsAndKZGProofs(missing=5)                          5         208400917 ns/op
PASS

[asn-d6]

LGTM! Thanks @cskiraly!

(Fixed a tiny nit with ea164c6 to unify the funcdoc with the rest of the file's style)