Initial progress at OP_MULTIPARAM

An initial attempt at implementing OP_MULTIPARAM by rewriting arg ops.
Handles mandatory, optional, and slurpy params.

Implement optional parameters with OP_PARAMTEST and OP_PARAMSTORE; use
the same SvPADSTALE flag trick that XS-Parse-Sublike uses

dump.c

@@ -1446,6 +1446,7 @@ S_do_op_dump_bar(pTHX_ I32 level, UV bar, PerlIO *file, const OP *o)
     case OP_ARGDEFELEM:
     case OP_ENTERTRY:
     case OP_ONCE:
+    case OP_PARAMTEST:
         S_opdump_indent(aTHX_ o, level, bar, file, "OTHER");
         S_opdump_link(aTHX_ o, cLOGOPo->op_other, file);
         break;
@@ -1563,6 +1564,35 @@ S_do_op_dump_bar(pTHX_ I32 level, UV bar, PerlIO *file, const OP *o)
         break;
     }
 
+    case OP_MULTIPARAM:
+    {
+        struct op_multiparam_aux *aux = (struct op_multiparam_aux *)cUNOP_AUXo->op_aux;
+        UV nparams = aux->params;
+        UV nparams_mandatory = nparams - aux->opt_params;
+        if(aux->opt_params)
+            S_opdump_indent(aTHX_ o, level, bar, file, "PARAMS = %" UVuf " .. %" UVuf "\n",
+                    nparams - aux->opt_params, nparams);
+        else
+            S_opdump_indent(aTHX_ o, level, bar, file, "PARAMS = %" UVuf "\n",
+                    nparams);
+
+        for(Size_t i = 0; i < nparams; i++) {
+            PADOFFSET padix = aux->param_padix[i];
+            if(padix)
+                S_opdump_indent(aTHX_ o, level, bar, file, "  PARAM [%zd] PADIX = %" UVuf "%s\n",
+                        i, aux->param_padix[i], i >= nparams_mandatory ? " OPT" : "");
+            else
+                S_opdump_indent(aTHX_ o, level, bar, file, "  PARAM [%zd] ANON\n",
+                        i);
+        }
+
+        if(aux->slurpy)
+            S_opdump_indent(aTHX_ o, level, bar, file, "SLURPY = '%c' PADIX = %" UVuf "\n",
+                    aux->slurpy, aux->slurpy_padix);
+
+        break;
+    }
+
     case OP_CUSTOM:
     {
         void (*custom_dumper)(pTHX_ const OP *o, struct Perl_OpDumpContext *ctx) =

op.h

@@ -1182,6 +1182,16 @@ struct op_argcheck_aux {
     char slurpy;     /* presence of slurpy: may be '\0', '@' or '%' */
 };
 
+/* for OP_MULTIPARAM */
+
+struct op_multiparam_aux {
+    UV    params;
+    UV    opt_params;
+    char  slurpy;
+    PADOFFSET *param_padix; /* points at storage allocated along with the struct itself, immediately following */
+    PADOFFSET slurpy_padix;
+};
+
 #define MI_INIT_WORKAROUND_PACK "Module::Install::DSL"