Some improvements to character literals

Do the backslash escape processing for MySQL compatible double quote mode.

Add E'...' as a Postgres compatible way of getting backslash escapes without having to enable that mode. This should also allow the ODBC driver to do client-side prepared statements.

If the literal has surrogates, its CHAR(n) type is the number of codepoints, not chars, as in the SQL layer.

In order to have tests of these, adjust the test framework to read the .expected files with UTF-8, not whatever the system default is.