Add Perl_bytes_cmp_utf8() to compare character sequences in different encodings

Add Perl_bytes_cmp_utf8() to compare character sequences in different encodings

Convert sv_eq_flags() and sv_cmp_flags() to use it.

Previously, to compare two strings of characters, where was was in UTF-8, and
one was not, you had to either:

1: Upgrade the second to UTF-8
2: Compare the resulting octet sequence
3: Free the temporary UTF-8 string

or:

1: Attempt to downgrade the first to bytes. If it can't be, they aren't equal
2: Else compare the resulting octet sequence
3: Free the temporary byte string

Which for the general case involves a malloc()/free() and at least two O(n)
scans per comparison.

Whereas this approach has no allocation, a single O(n) scan, which terminates
as early as the best case for the second approach.