Fix destructuring alternation

Attempting to use the existing FORK_OPT opcode resulted in difficulty
knowing when to pop an error message off the stack and when not to. This
commit makes DESTRUCTURE_ALT a real opcode that is identical to
FORK_OPT, except for never pushing the error message onto the stack when
continuing from an error backtrack.

Some small changes were necessary to the DUP/POP behavior surrounding
destructuring to accomodate this.

src/opcode_list.h

@@ -44,5 +44,5 @@ OP(DEPS, CONSTANT, 0, 0)
 OP(MODULEMETA, CONSTANT, 0, 0)
 OP(GENLABEL, NONE, 0, 1)
 
-OP(DESTRUCTURE_ALT, NONE, 0, 0)
+OP(DESTRUCTURE_ALT, BRANCH, 0, 0)
 OP(STOREVN, VARIABLE, 1, 0)