add partial support for int128

[mo-xiaoming]

For the moment, hobbes doesn't handle int128 type well if the value is not within the range of int64

> 9223372036854775808H         # int64::max + 1
-9223372036854775808
> -9223372036854775809H        # int64::min - 1
9223372036854775807

After this change, int128 still won't work with int128::min because parser rewrites negative number to a neg function call. For example, -3 -> neg(3). However, for any integral types, abs(min) overflows. This issue will be addressed in a different PR