SHA256 checksums

[Jeremy Lavergne]

Following up on a previous thread conversation [1], I'd like to know if
we're remotely interested in offering multi-length SHA256 checksums. This
will allow us to have SHA256 checksums of a more desirable length while
automatically switching between the bases.

Specifically, inside the portchecksum::checksum_main function we would
check the length of the existing checksum to provide an optional argument
to portchecksum::calc_256 to switch which the base of checksum it returns.

Apparently afb already submitted base32 encode/decode to pextlib.

 [1]
http://lists.macosforge.org/pipermail/macports-dev/2010-October/013070.html