Add i128 and u128 Support for Integer ASTs

[mehrad31415]

Add i128 and u128 Support for Integer ASTs

This PR extends the Int<'ctx> API to support 128-bit integers.

• from_i128(ctx: &Context, v: i128) -> Int<'ctx>
  Creates a Z3 integer numeral from a signed 128-bit integer.

• from_u128(ctx: &Context, v: u128) -> Int<'ctx>
  Creates a Z3 integer numeral from an unsigned 128-bit integer.

• as_i128(&self) -> Option<i128>
  Attempts to parse the internal numeral into a signed i128. Returns None if the AST is not a numeral or parsing fails.

• as_u128(&self) -> Option<u128>
  Same as above, but for unsigned u128.

[toolCHAINZ]

I'll look at merging this in post-#410 because there's some overlap there and I don't want to fix the merge twice.