Removed internal cache for closures

The mechanism of "caching the last closure created for a prototype to
try to reuse it the next time a closure for that prototype is created"
was removed. There are several reasons:

- It is hard to find a natural example where this cache has a measurable
impact on performance.

- Programmers already perceive closure creation as something slow,
so they tend to avoid it inside hot paths. (Any case where the cache
could reuse a closure can be rewritten predefining the closure in some
variable and using that variable.)

- The implementation was somewhat complex, due to a bad interaction
with the generational collector. (Typically, new closures are new,
while prototypes are old. So, the cache breaks the invariant that
old objects should not point to new ones.)

lstate.h

@@ -43,8 +43,6 @@
 ** 'weak': tables with weak values to be cleared;
 ** 'ephemeron': ephemeron tables with white->white entries;
 ** 'allweak': tables with weak keys and/or weak values to be cleared.
-** There is also a list 'protogray' for prototypes that need to have
-** their caches cleared.
 
 */
 
@@ -186,7 +184,6 @@ typedef struct global_State {
   GCObject *weak;  /* list of tables with weak values */
   GCObject *ephemeron;  /* list of ephemeron tables (weak keys) */
   GCObject *allweak;  /* list of all-weak tables */
-  GCObject *protogray;  /* list of prototypes with "new" caches */
   GCObject *tobefnz;  /* list of userdata to be GC */
   GCObject *fixedgc;  /* list of objects not to be collected */
   /* fields for generational collector */