This commit adds the json_escape_forward_slash variable and a tweak for
it that is now used by the compat module. The new variable configures
whether '/' is escaped when encoded in JSON.

Note, the old implementation was quite messy so we had to rework it:

 - Drop luaL_serializer::encode_escape_forward_slash. This was an
   undocumented serializer option implemented only by the JSON
   serializer and used only by the compat module. Now, we use the
   json_escape_forward_slash global tweak for all JSON serializers
   instead, because making this tweak configurable per serializer
   doesn't make much sense.

 - Don't use mp_char2escape for escaping JSON characters. Historically,
   we used this table defined in libmsgpuck in the json_escape utility
   function. It equals the escape table used by the JSON encoder so it
   looks more reasonable to use the JSON encoder escape table in
   json_escape. This commit moves the JSON encoder escape table to
   util.c and adds an inline utility function json_escape_char which is
   now used by the JSON encoder and json_escape.

 - Drop an extra JSON escape table with '/' escaped. We had two escape
   tables in JSON, one with escaped '/' and another with unescaped '/'.
   Duplicating tables like this is error-prone and ugly. Let's use one
   table with '/' unescaped and check the json_escape_forward_slash flag
   in json_escape_char instead. The cost of this check is negligible
   performance-wise.

This commit also drops the lua/compat.c source file, because it isn't
used anymore.

While we are at it, remove any mentions of MsgPack from the changelog
entry for the json_escape_forward_slash compat option, because it isn't
relevant anymore.

Closes #8117

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring