proto: activate sub inlining without () proto

See [cperl #87]
We have two ways to constant fold subs.
fold to a fast CONSTSUB without entersub
and fold to a dummy XS call with returning the value immediately.

there’s no semantic visibility of those 3 variants, other than the required
() proto or the usage of use constant pragma.  there’s no semantic
distinction of constant folded to CONSTSUB, dummy_xs or not constant folded
subs, other than improved run-time performance and a different internal
representation.

dropping the requirement to use the () seems
1. fair, and
2. even other prototypes should be allowed to fold
if those parameters are ignored in the body, when at compile-time the body
folds to a constant sv.

WIP: but I am getting prototype mismatches

CHANGE: set the proto of the created dummy XS to "" or "$" if a method.

op.c

@@ -11561,7 +11561,17 @@ Perl_newMYSUB(pTHX_ I32 floor, OP *o, OP *proto, OP *attrs, OP *block)
 	    CvSTASH_set(cv, PL_curstash);
 	    *spot = cv;
 	}
-        SvPVCLEAR(MUTABLE_SV(cv));  /* prototype is "" */
+        if (*ps) {
+            SvPV_set(MUTABLE_SV(cv), ps);
+            SvCUR_set(MUTABLE_SV(cv), ps_len);
+            if (ps_utf8)
+                SvUTF8_on(MUTABLE_SV(cv));
+        }
+        else
+            if (CvMETHOD(cv))
+                sv_setpvs(MUTABLE_SV(cv), "$");  /* prototype for $self */
+            else
+                SvPVCLEAR(MUTABLE_SV(cv));  /* prototype is "" */
 	CvXSUBANY(cv).any_ptr = const_sv;
 	CvXSUB(cv) = S_const_sv_xsub;
 	CvCONST_on(cv);
@@ -12045,7 +12055,7 @@ Perl_newATTRSUB_x(pTHX_ I32 floor, OP *o, OP *proto, OP *attrs,
     }
 
     if (!block
-#ifndef USE_CPERL_not_yet
+#ifndef USE_CPERL
         /* allow inlining of constant bodies on cperl even without empty proto*/
         || !ps || *ps /* perl5: sub x{1} => no proto, so not inlinable */
 #endif
@@ -12147,7 +12157,17 @@ Perl_newATTRSUB_x(pTHX_ I32 floor, OP *o, OP *proto, OP *attrs,
                the any_ptr as value*/
 	    assert(!CvROOT(cv) && !CvCONST(cv));
 	    cv_forget_slab(cv);
-            SvPVCLEAR(MUTABLE_SV(cv));  /* prototype is "" */
+            if (*ps) {
+                SvPV_set(MUTABLE_SV(cv), ps);
+                SvCUR_set(MUTABLE_SV(cv), ps_len);
+                if (ps_utf8)
+                    SvUTF8_on(MUTABLE_SV(cv));
+            }
+            else
+                if (CvMETHOD(cv))
+                    sv_setpvs(MUTABLE_SV(cv), "$");  /* prototype for $self */
+                else
+                    SvPVCLEAR(MUTABLE_SV(cv));  /* prototype is "" */
 	    CvXSUBANY(cv).any_ptr = const_sv;
 	    CvXSUB(cv) = S_const_sv_xsub;
 	    CvCONST_on(cv);