Add alternation destructuring operator ?//

This is a first pass to show the implementation.
It needs tests and evaluation, but doesn't break any existing tests.

NOT READY FOR MERGING

src/opcode_list.h

@@ -2,6 +2,7 @@ OP(LOADK, CONSTANT, 1, 1)
 OP(DUP,   NONE,     1, 2)
 OP(DUPN,  NONE,     1, 2)
 OP(DUP2,  NONE,     2, 3)
+OP(PUSHK_UNDER, CONSTANT, 1, 2)
 OP(POP,   NONE,     1, 0)
 OP(LOADV, VARIABLE, 1, 1)
 OP(LOADVN, VARIABLE, 1, 1)
@@ -42,3 +43,6 @@ OP(CLOSURE_PARAM_REGULAR, DEFINITION, 0, 0)
 OP(DEPS, CONSTANT, 0, 0)
 OP(MODULEMETA, CONSTANT, 0, 0)
 OP(GENLABEL, NONE, 0, 1)
+
+OP(DESTRUCTURE_ALT, NONE, 0, 0)
+OP(STOREVN, VARIABLE, 1, 0)