Changed shootout-fasta-redux to use size_t when calling fwrite, removed XFAIL
auto merge of #5989 : Thiez/rust/fixbench, r=catamorphism

Partial fix for #5985
shootout-fasta-redux.rs was calling fwrite with u64 arguments that should have been size_t, which broke on 32-bit systems. I replaced the casts to u64 by casts to size_t.

r?