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/parser.y

@@ -83,6 +83,7 @@ struct lexer_param;
 %token SETDEFINEDOR "//="
 %token LESSEQ "<="
 %token GREATEREQ ">="
+%token ALTERNATION "?//"
 
 %token QQSTRING_START
 %token <literal> QQSTRING_TEXT
@@ -118,7 +119,7 @@ struct lexer_param;
 %type <blk> FuncDef FuncDefs
 %type <blk> Module Import Imports ImportWhat ImportFrom
 %type <blk> Param Params Arg Args
-%type <blk> Pattern ArrayPats ObjPats ObjPat
+%type <blk> Patterns RepPatterns Pattern ArrayPats ObjPats ObjPat
 %type <literal> Keyword
 %{
 #include "lexer.h"
@@ -176,10 +177,6 @@ static jv check_object_key(block k) {
   return jv_invalid();
 }
 
-static block gen_dictpair(block k, block v) {
-  return BLOCK(gen_subexp(k), gen_subexp(v), gen_op_simple(INSERT));
-}
-
 static block gen_index(block obj, block key) {
   return BLOCK(gen_subexp(key), obj, gen_op_simple(INDEX));
 }
@@ -342,19 +339,18 @@ FuncDef Exp %prec FUNCDEF {
   $$ = block_bind_referenced($1, $2, OP_IS_CALL_PSEUDO);
 } |
 
-Term "as" Pattern '|' Exp {
+Term "as" Patterns '|' Exp {
   $$ = gen_destructure($1, $3, $5);
 } |
-
-"reduce" Term "as" Pattern '(' Exp ';' Exp ')' {
+"reduce" Term "as" Patterns '(' Exp ';' Exp ')' {
   $$ = gen_reduce($2, $4, $6, $8);
 } |
 
-"foreach" Term "as" Pattern '(' Exp ';' Exp ';' Exp ')' {
+"foreach" Term "as" Patterns '(' Exp ';' Exp ';' Exp ')' {
   $$ = gen_foreach($2, $4, $6, $8, $10);
 } |
 
-"foreach" Term "as" Pattern '(' Exp ';' Exp ')' {
+"foreach" Term "as" Patterns '(' Exp ';' Exp ')' {
   $$ = gen_foreach($2, $4, $6, $8, gen_noop());
 } |
 
@@ -778,6 +774,22 @@ Exp {
   $$ = gen_lambda($1);
 }
 
+RepPatterns:
+RepPatterns "?//" Pattern {
+  $$ = BLOCK($1, gen_destructure_alt($3));
+} |
+Pattern {
+  $$ = gen_destructure_alt($1);
+}
+
+Patterns:
+RepPatterns "?//" Pattern {
+  $$ = BLOCK($1, $3);
+} |
+Pattern {
+  $$ = $1;
+}
+
 Pattern:
 '$' IDENT {
   $$ = gen_op_unbound(STOREV, jv_string_value($2));