pgsql: Revert refactoring of hex code to src/common/

Revert refactoring of hex code to src/common/

This is a combined revert of the following commits:
- c3826f8, a refactoring piece that moved the hex decoding code to
src/common/. This code was cleaned up by aef8948, 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.
- aef8948, 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.
- ccf4e27, a cleanup made possible by aef8948.

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

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/2576dcfb76aa71e4222bac5a3a43f71875bfa9e8

Modified Files
--------------
src/backend/replication/backup_manifest.c | 30 ++---
src/backend/utils/adt/encode.c | 158 +++++++++++++++---------
src/backend/utils/adt/varlena.c | 15 +--
src/common/Makefile | 1 -
src/common/hex.c | 192 ------------------------------
src/include/common/hex.h | 25 ----
src/include/common/sha2.h | 4 +
src/include/utils/builtins.h | 4 +
src/tools/msvc/Mkvcbuild.pm | 2 +-
9 files changed, 127 insertions(+), 304 deletions(-)