Limit internal buffer size for indexed FASTAs.
Limit internal buffer size for indexed FASTAs.

This patch limits the internal buffer used when reading indexed
FASTA files to 512 bytes. Previously the buffer size would match
the line-length, potentially resulting in very large buffers being
created when reading small intervals.

pmarks/rust-bio,FrancisRussell/rust-bio,rust-bio/rust-bio