Stream-based handling of FASTA files. The user selects a window
size, the library then handles the window. For each window, the
previous (past) window is available, in case some data sits on
the boundary between windows.
Greg Schwartz' http://hackage.haskell.org/package/fasta
package is a lot more complete. This one is mostly tailored to
my usage requirements (and may at some point use his library).