Deprecate use of := to mean an empty attribute list in my $pi := 4;

Deprecate use of := to mean an empty attribute list in my $pi := 4;

An accident of Perl's parser meant that my $pi := 4; was parsed as an empty
attribute list. Empty attribute lists are ignored, hence the above is
equivalent to my $pi = 4; However, the fact that it is currently valid syntax
means that := cannot be used as new token, without silently changing the
meaning of existing code.

Hence it is now deprecated, so that it can subsequently be removed, allowing
the possibility of := to be used as a new token with new semantics.