Fill protos tweaks

I got another branch:

 1. Fatal: Optimize out calls to fill_protos
 2. Fatal: special case "@" prototypes in fill_protos

Commit 1. is a minor optimization of load time for importing of cached subs.  It does put a bit of extra cost on generating the trampoline for leak calls to CORE subs.  That said I believe it overall is a win because you now pay for "fill_protos" at most twice per sub (once at import and once at the first leak) instead of once per import of the sub.

Commit 2. makes fill_protos special case prototypes consisting entirely of "@" (or ";@") and have it emit a simpler call by using @_ instead of slicing it as @_[0..$#_].