Add test cases for malformed _nat literals

[saxbophone]

• Test cases for use of digits outside those permissible for a given base, e.g:
  • 102FFCDEAB0_nat (hex digits used when the literal is meant to be decimal)
  • 0xZZt3F_29313_nat (digits outside of hex literal)
• Modify user-defined-literal code to raise an error when these are encountered