UCD.pm: Add check for parameter type

[khwilliamson]

This needs to be a scalar reference; I got burned with it not doing the right thing when it wasn't.

• I'm unsure if this set of changes requires a perldelta entry

[tonycoz]

This could use a test.

• I'm unsure if this set of changes requires a perldelta entry, and it is included.

I don't see the perldelta entry.

[jkeenan]

This could use a test.

Provided one at khwilliamson#1

[tonycoz]

The test made obvious to me:

tony@venus:.../git/perl6$ ./perl -Ilib -MUnicode::UCD=num -e 'my $x; num("123", \$x)'
tony@venus:.../git/perl6$ ./perl -Ilib -MUnicode::UCD=num -e 'my $x=\1; num("123", \$x)'
Unicode::UCD::num: second parameter must be a scalar reference at -e line 1.

In both cases the parameter is a reference to $x, and $$x can be assigned to.

Another case would be:

tony@venus:.../git/perl6$ ./perl -Ilib -MUnicode::UCD=num -E 'my $x; num("123", bless \$x);'
Unicode::UCD::num: second parameter must be a scalar reference at -e line 1.

But I think that's more a case of "Doctor, it hurts when I do (something dumb)"

Alas builtin::reftype() also returns REF for the second case.

I'm not fond of this ref() behaviour (ref() -> REF describes the contents of the reference container, not the referenced container itself)