Parser: add support for unicode space characters

[Mytherin]

This PR adds support for unicode space characters in the parser. These characters otherwise cause confusing errors - as seemingly correct SQL statements will end up resulting in syntax errors, e.g.:

D SELECT 42;
Error: Parser Error: syntax error at or near "SELECT 42"
LINE 1: SELECT 42;

Unfortunately the current flex lexer does not have good support for unicode - and does not work nicely when we need multiple byte look-ahead (which we need for the various 3-byte unicode space characters). Instead, we add this support by performing a separate pass prior to parsing that converts unicode spaces into regular spaces.