Experimental implementation of infix subroutines.

This commit introduces a new prototype, >, which allows
creating infix subroutines.

    sub infix ($>$) { say "<@_>" }

    10 infix $foo;

When declaring infix subs, the @ and % prototypes have special
meaning:

   sub array_hash (@>%) { ... } # $_[0] will be an arrayref
                                # $_[1] will be a hashref

   (1, 2, 3, 4) array_hash map { $_ => 1 } qw/a b c d/;

This feature in a work in progress!