Add inlinable &CORE::functions

Add inlinable &CORE::functions

This commit allows this to work:

  BEGIN { *entangle = \&CORE::tie };
  entangle $foo, $package;

And the entangle call gets inlined as a tie op, the resulting op tree
being indistinguishable.

These subs are not yet callable via &foo syntax or through a refer-
ence.  That will come later, except for some functions, like sort(),
which will probably never support it.

Almost all overridable functions are supported.  These few are not:

  - infix operators
  - not and getprotobynumber (can’t get the precedence right yet;
    prototype problem)
  - dump

Subsequent commits (hopefully!) will deal with those.

How this works:

gv_fetchpvn_flags is extended with hooks to create subs inside the
CORE package.  Those subs are XSUBs (whose C function dies with an
error, for now at least) with a call checker that blows away the
entersub op and replaces it with whatever op the sub represents.

This is slightly inefficient right now, as gv_fetchpvn_flags calls
keyword(), only to have core_prototype call it again.  That will
be fixed in a future refactoring.