FIX: Bitset not allowed in to/thru block

Allow to/thru [bitset|...] in PARSE
For example:
```
>> x: charset "x" y: charset "y" parse "y" [thru [x | y]]
== true
```

Fixes: metaeducation/rebol-issues#2141

(cherry picked from commit 68a0e0a)

src/core/u-parse.c

@@ -435,6 +435,11 @@ void Print_Parse_Index(REBCNT type, REBVAL *rules, REBSER *series, REBCNT index)
  }
  }
  }
+ // bitset
+ else if (IS_BITSET(item)) {
+ if (Check_Bit(VAL_SERIES(item), ch1, !HAS_CASE(parse)))
+ goto found1;
+ }
  else if (ANY_STR(item)) {
  ch2 = VAL_ANY_CHAR(item);
  if (!HAS_CASE(parse)) ch2 = UP_CASE(ch2);