Adding support for integers

Changes:
- ConsValue changed to support integer literals
- Parser and lexer changed to support integer literals
- Added 'int?' and 'float?' built-ins for type testing
- Math and comparison built-ins rewritten to support both integers and floats
- Fixed bug where '-' called with one argument returned that argument
- '/' can now be called with one argument, or more than two arguments
- Added internal numeric equality function for use with future '==' stdlib function