Checksum Improvements

Just a quick update, some checksum improvements have been contributed to Factor.

Some new checksums have been implemented:

• checksums.murmur: MurmurHash3 in 32-bit
• checksums.fletcher: Fletcher in 16/32/64-bits
• checksums.superfast: SuperFastHash
• checksums.xxhash: xxHash in 32-bit

And some checksum performance has been improved:

• checksums.md5 is a lot faster (benchmark is 0.080 vs 0.583 seconds)
• checksums.sha is a bit faster (benchmark is 0.418 vs 0.686 seconds)

You can find these changes (and more!) in the development version of Factor.