Stop lexical CORE sub from interfering with CORE::

Stop lexical CORE sub from interfering with CORE::

The way CORE:: was handled in the lexer was convoluted.

CORE was treated initially as a keyword, with exceptions in the lexer
to make it behave correctly.  If it turned out not to be followed
by ::, then the lexer would fall back to treating it as a bareword
or sub name.

Before even checking for a keyword, the lexer looks for :: and goes
to the bareword/sub code.  But it made a special exception there
for CORE::.

In the end, treating CORE as a keyword recognized by the keyword()
function requires more special cases than simply special-casing CORE::
in toke.c.

This fixes the lexical CORE sub bug, while reducing the total num-
ber of lines.