Fasta reader tests

[joshfactorial]

in fasta_reader/src/lib.rs - There are some basic tests, but they are very baseline. The second test, on read_fasta only checks the result of the first element of the array. More tests might be good. Gather stats and build contig map are not fully tested. working toward 100% coverage for this core feature is the goal. Any comments or code improvements along the way are welcome.