Avoid a closure per NFA evaluation.

We achieve this by a re-think of how we work with the NFA cache. Needs
a patch to the Rakudo MOP also for its regexes/grammars to work with
this change. For CORE.setting this saves a dozen GCs from the code
objects produced alone.

Author: jnthn

src/QRegex/Cursor.nqp

@@ -306,7 +306,11 @@ role NQPCursorRole is export {
     method !protoregex($name) {
         # Obtain and run NFA.
         my $shared := $!shared;
-        my $nfa := self.HOW.cache(self, $name, { self.'!protoregex_nfa'($name) });
+        my $nfa    := self.HOW.cache_get(self, $name);
+        if nqp::isnull($nfa) {
+            $nfa := self.'!protoregex_nfa'($name);
+            self.HOW.cache_add(self, $name, $nfa);
+        }
         my @fates := $nfa.run(nqp::getattr_s($shared, ParseShared, '$!target'), $!pos);
 
         # Update highwater mark.
@@ -367,7 +371,11 @@ role NQPCursorRole is export {
         }
 
         # Evaluate the alternation.
-        my $nfa := self.HOW.cache(self, $name, { self.'!alt_nfa'($!regexsub, $name) });
+        my $nfa := self.HOW.cache_get(self, $name);
+        if nqp::isnull($nfa) {
+            $nfa := self.'!alt_nfa'($!regexsub, $name);
+            self.HOW.cache_add(self, $name, $nfa);
+        }
         $nfa.run_alt(nqp::getattr_s($shared, ParseShared, '$!target'), $pos, $!bstack, $!cstack, @labels);
     }
 
@@ -395,7 +403,7 @@ role NQPCursorRole is export {
         sub precomp_alt_nfas($meth) {
             if nqp::can($meth, 'ALT_NFAS') {
                 for $meth.ALT_NFAS -> $name {
-                    self.HOW.cache(self, $name, { self.'!alt_nfa'($meth, $name.key) });
+                    self.HOW.cache(self, ~$name, { self.'!alt_nfa'($meth, $name.key) });
                 }
             }
         }

src/how/NQPClassHOW.nqp

@@ -734,6 +734,7 @@ knowhow NQPClassHOW {
     ##
     ## Cache-related
     ##
+
     method cache($obj, $key, $value_generator) {
         %!caches := nqp::hash() unless nqp::ishash(%!caches);
         nqp::existskey(%!caches, $key) ??
@@ -746,7 +747,17 @@ knowhow NQPClassHOW {
         %!caches := {} unless nqp::isnull(%!caches);
         nqp::scwbenable();
     }
-    
+
+    method cache_get($obj, $key) {
+        my %caches := %!caches;
+        nqp::ishash(%caches) ?? nqp::atkey(%caches, $key) !! nqp::null()
+    }
+
+    method cache_add($obj, $key, $value) {
+        %!caches := nqp::hash() unless nqp::ishash(%!caches);
+        %!caches{$key} := $value;
+    }
+
     ##
     ## Mix-ins
     ##