Optimize escaping of JSON strings

There were quite a few places where we either had a non-NUL-terminated
string or a text Datum which we needed to call escape_json() on.  Many of
these places required that a temporary string was created due to the fact
that escape_json() needs a NUL-terminated cstring.  For text types, those
first had to be converted to cstring before calling escape_json() on them.

Here we introduce two new functions to make escaping JSON more optimal:

escape_json_text() can be given a text Datum to append onto the given
buffer.  This is more optimal as it foregoes the need to convert the text
Datum into a cstring.  A temporary allocation is only required if the text
Datum needs to be detoasted.

escape_json_with_len() can be used when the length of the cstring is
already known or the given string isn't NUL-terminated.  Having this
allows various places which were creating a temporary NUL-terminated
string to just call escape_json_with_len() without any temporary memory
allocations.

Discussion: https://postgr.es/m/CAApHDvpLXwMZvbCKcdGfU9XQjGCDm7tFpRdTXuB9PVgpNUYfEQ@mail.gmail.com
Reviewed-by: Melih Mutlu, Heikki Linnakangas

src/backend/utils/adt/jsonb.c

@@ -354,7 +354,7 @@ jsonb_put_escaped_value(StringInfo out, JsonbValue *scalarVal)
 			appendBinaryStringInfo(out, "null", 4);
 			break;
 		case jbvString:
-			escape_json(out, pnstrdup(scalarVal->val.string.val, scalarVal->val.string.len));
+			escape_json_with_len(out, scalarVal->val.string.val, scalarVal->val.string.len);
 			break;
 		case jbvNumeric:
 			appendStringInfoString(out,