This is a combined revert of the following commits:
- c3826f83, a refactoring piece that moved the hex decoding code to
src/common/.  This code was cleaned up by aef8948f, as it originally
included no overflow checks in the same way as the base64 routines in
src/common/ used by SCRAM, making it unsafe for its purpose.
- aef8948f, a more advanced refactoring of the hex encoding/decoding code
to src/common/ that added sanity checks on the result buffer for hex
decoding and encoding.  As reported by Hans Buschmann, those overflow
checks are expensive, and it is possible to see a performance drop in
the decoding/encoding of bytea or LOs the longer they are.  Simple SQLs
working on large bytea values show a clear difference in perf profile.
- ccf4e277, a cleanup made possible by aef8948f.

The reverts of all those commits bring back the performance of hex
decoding and encoding back to what it was in ~13.  Fow now and
post-beta3, this is the simplest option.

Reported-by: Hans Buschmann
Discussion: https://postgr.es/m/1629039545467.80333@nidsa.net
Backpatch-through: 14