First stab at lexical scalar aliases

First stab at lexical scalar aliases

No \my$x= yet.  Only my $x; \$x =....

It does not work properly with variables closed over from outside;
hence, all the to-do tests fail still, since they do the assign-
ment in evals.

But this much works:

$ ./miniperl -Ilib -Mfeature=:all -e 'my $m; \$m = \$n; warn \$m; warn \$n'
Lvalue references are experimental at -e line 1.
SCALAR(0x7fa04b805510) at -e line 1.
SCALAR(0x7fa04b805510) at -e line 1.