Fix UTF-8 support in placeholders and return values
Add support for UTF-8 statement strings
Fix quoting of UTF-8 values
Test pg_enable_utf8 of -1, in addittion to 0 and 1.
Refactor Unicode test to use anon hashes to describe the tests to run.

Rename the desired return value as $want, and there's no saving in assigning
to $t for just one test.
Test that the Pg server agrees with us about the lengths of input strings.

This should stop us missing bugs where we send a string encoded in some
bogus form, and accurately reverse the transformation on return. If we
happen to have forwards and backwards bogosity exactly matched, then the
Perl would think that it was communicating correctly because it got the same
answer back, and wouldn't be aware of the mistake.
Extend the Unicode round-trip tests to verify ASCII, BMP and non-BMP code points.

Test that everything round trips, and that the Pg server has the same idea
of string length as we do.
Test that characters created in the server reach the client correctly.