Hex string literal x'...' incorrectly parsed as keyword + string #649

Bug

The lexer incorrectly tokenizes hex string literals x'...' as two separate tokens (keyword x + string '...'),
causing the builder to produce invalid SQL x AS '...' instead of x'...'.

This affects CREATE TABLE statements with binary column defaults on MariaDB 11.8+ which outputs DEFAULT x'...'
in SHOW CREATE TABLE.

The b'...' binary string literal format is handled correctly — only x'...' is affected.

To Reproduce

  $parser = new Parser("CREATE TABLE test (IP binary(16) NOT NULL DEFAULT x'00000000000000000000000000000000')");
  echo $parser->statements[0]->build();                                                                               
  // Output: ... DEFAULT x AS `00000000000000000000000000000000`                                                      
  // Expected: ... DEFAULT x'00000000000000000000000000000000'

Affected versions

Both 5.11.x and master.

Root cause

The Lexer's number parsing state machine has states 7-9 for b'...' binary literals, but no equivalent states for
x'...' hex string literals. The x is instead matched as a keyword.