Avoid creating GVs when subs are declared

This patch changes ‘sub foo {...}’ declarations to store subroutine
references in the stash, to save memory.

Typeglobs still notionally exist.  Accessing CvGV(cv) will reify them.
Hence, currently the savings are lost when a sub call is compiled.

$ ./miniperl -e 'sub foo{} BEGIN { warn $::{foo} } foo(); BEGIN { warn $::{foo} }'
CODE(0x7f8ef082ad98) at -e line 1.
*main::foo at -e line 1.

This optimisation is skipped if the subroutine declaration contains a
package separator.

Concerning the changes in caller.t, this code:

    sub foo { print +(caller(0))[3],"\n" }
    my $fooref = delete $::{foo};
    $fooref -> ();

used to crash in 5.7.3 or thereabouts.  It was fixed by 16658 (aka
07b8c804e8) to produce ‘(unknown)’ instead.  Then in 5.13.3 it was
changed (by 803f274) to produce ‘main::__ANON__’ instead.  So the
tests are really checking that we don’t get a crash.  I think it is
acceptable that it has now changed to ‘main::foo’.

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