Address #547 - don't drop references.

Keep a separate stack for tagged references. May cause pegs to
use more memory but makes the backref and backmatch features much more
powerful.

Also disables the second stack if backref and backmatch are not used in the peg.

src/include/janet.h

@@ -1839,6 +1839,7 @@ typedef struct {
     Janet *constants;
     size_t bytecode_len;
     uint32_t num_constants;
+    int has_backref;
 } JanetPeg;
 
 #endif