Rearrange integer literal tests and add some new testcases.

This moves the integer literal tests to the AST tests, which has the
advantage of not requiring a TestHost just to parse a constant
expression.
Fix long hex literals.

Previously, hex literals that were outside the range of an integer
were always parsed as 0 due to overflow.
Implement numeric multiplier for all integer literal types.

The numeric multiplier now works for decimal/double integer literals
and correctly handles type conversions due to overflow for all literal
types.
Add support for decimal integer literals.