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.

1 files changed, 18 insertions, 4 deletions

diff --git a/lib/CORE.pod b/lib/CORE.pod
index b96c1de2d6..d2175eb56c 100644
--- a/lib/CORE.pod
+++ b/lib/CORE.pod
@@ -1,6 +1,6 @@
 =head1 NAME
 
-CORE - Pseudo-namespace for Perl's core routines
+CORE - Namespace for Perl's core routines
 
 =head1 SYNOPSIS
 
@@ -12,17 +12,31 @@ CORE - Pseudo-namespace for Perl's core routines
     print CORE::hex("0x50"),"\n";		# prints 80
     CORE::say "yes";				# prints yes
 
+    BEGIN { *shove = \&CORE::push; }
+    shove @array, 1,2,3;			# pushes on to @array
+
 =head1 DESCRIPTION
 
 The C<CORE> namespace gives access to the original built-in functions of
-Perl.  It also provides access to keywords normally available
-only through the L<feature> pragma.  There is no C<CORE>
-package, and therefore you do not need to use or
+Perl.  The C<CORE> package is built into
+Perl, and therefore you do not need to use or
 require an hypothetical "CORE" module prior to accessing routines in this
 namespace.
 
 A list of the built-in functions in Perl can be found in L<perlfunc>.
 
+For all Perl keywords, a C<CORE::> prefix will force the built-in function
+to be used, even if it has been overridden or would normally require the
+L<feature> pragma.  Despite appearances, this has nothing to do with the
+CORE package, but is part of Perl's syntax.
+
+For many Perl functions, the CORE package contains real subroutines.  This
+feature is new in Perl 5.16.  You can take references to these and make
+aliases.  However, they can only be called as barewords; i.e., you cannot
+use ampersand syntax (C<&foo>) or call them through references.  See the
+C<shove> example above.  This works for all overridable keywords, except
+for C<dump>, C<getprotobynumber>, C<not> and the infix operators.
+
 =head1 OVERRIDING CORE FUNCTIONS
 
 To override a Perl built-in routine with your own version, you need to