toke.c: Removed the utf16 filter

Instead, now we push a ":raw:encoding(UTF-16[LB]E)" layer on the
parser's filehandle, and additionally a :crlf layer after those
if PERLIO_USING_CRLF is defined.

This involved getting rid of the PERL_NO_UTF16_FILTER symbol,
and moving t/comp/utf.t to t/uni/utf.t, since that test now
requires both PerlIO and Encode.

Additionally, this introduces one further incompatibility:
The original filter would silently accept an improperly encoded
newlines in the UTF-16 source, whereas the encoding layer now
warns about them as partial characters.