optimize md5_text

This patch optimizes the md5_text() function (which is used to implement
the md5() SQL-level function). The old code did the following:

1. de-toast the datum
2. convert it to a cstring via textout()
3. get the length of the cstring via strlen()

Since we are treating the datum context as a blob of binary data, the
latter two steps are unnecessary. Once the data has been detoasted, we
can just use it as-is, and derive its length from the varlena metadata.

This patch improves some run-of-the-mill md5() computations by just
under 10% in my limited tests, and passes the regression tests.

I also noticed that md5_text() wasn't checking the return value of
md5_hash(); encountering OOM at precisely the right moment could result
in returning a random md5 hash. This patch corrects that. A better fix
would be to make md5_hash() only return on success (and/or allocate via
palloc()), but since it's used in the frontend as well I don't see an
easy way to do that.

Barring any objections, I'll apply this to HEAD tomorrow.

-Neil