bench: add internal benchmark for `secp256k1_fe_normalize_var` #1855

While addressing the review suggestion #1765 (review) (b10c mirror link), I noticed that we don't have an internal benchmark for the variable-time variant of _fe_normalize yet, so this PR adds one. IIUC it's fine to repeatedly apply the operation on the same (already normalized at latest after the first loop iteration) field element for benchmarking purposes and don't put in an effort to reach the final reduction code path, considering how extremely unlikely it is to reach it in practice.

Results on my machine:

$ ./build/bin/bench_internal normalize
Benchmark                     ,    Min(us)    ,    Avg(us)    ,    Max(us)    

field_normalize               ,     0.0103    ,     0.0106    ,     0.0128 
field_normalize_var           ,     0.00545   ,     0.00546   ,     0.00547
field_normalize_weak          ,     0.00352   ,     0.00354   ,     0.00363

ACK 8b066a4dd772e6a4d3d3cc1a48b0683214654c23

ACK 8b066a4dd772e6a4d3d3cc1a48b0683214654c23

Worth noting that this benchmarks the optimistic path? It makes sense to do so, because the worst-case path is very unlikely to be needed.

Worth noting that this benchmarks the optimistic path? It makes sense to do so, because the worst-case path is very unlikely to be needed.

Makes sense, thanks. Added a corresponding comment.