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

src/tools/msvc/Mkvcbuild.pm

@@ -125,7 +125,7 @@ sub mkvcbuild
 	our @pgcommonallfiles = qw(
 	  archive.c base64.c checksum_helper.c
 	  config_info.c controldata_utils.c d2s.c encnames.c exec.c
-	  f2s.c file_perm.c file_utils.c hashfn.c hex.c ip.c jsonapi.c
+	  f2s.c file_perm.c file_utils.c hashfn.c ip.c jsonapi.c
 	  keywords.c kwlookup.c link-canary.c md5_common.c
 	  pg_get_line.c pg_lzcompress.c pgfnames.c psprintf.c relpath.c rmtree.c
 	  saslprep.c scram-common.c string.c stringinfo.c unicode_norm.c username.c