Add support for more Int types & Refactor files

[JustinReiter]

Simplifies the logic for creating Integer types for call LLVM instructions and LLVM Arithmetic intrinsic functions. Any signed integer type that is 64 bits and below is currently supported (i8, i16, i32, i64 and isize).

TODO: add support for unsigned type. In theory, this should be very similar to signed integers (requires modification of constraints and z3 variables).

Update: support for unsigned types are trickier than expected. LLVM does not preserve the type and lifts all unsigned to be signed values. Intrinsic functions still use the unsigned counterparts, but use signed arguments (and handles the operations correctly). There still seems to be a notion of unsigned type when running the program, but unclear where this is from. (Look at README for a stack overflow article)

Testing framework also updated to create the temp test directory if it does not already exist (otherwise there will be a panic).

TODO: update the temp test dir logic to be deleted afterwards (possibly).

Update: Added new tests for each signed type that is currently supported (i8, i16, i32 & i64). i128 is not currently implemented due to the Z3 binding working with values up to i64 easily (whereas, a different conversion to a Z3 Int would be needed for larger int types).

Lastly, files and modules refactored into a nested structure. Lib and imports have been rearranged.

[wu-benjamin]

Looks good to me for the most part!

It would be nice to use is_int_type when constraining inputs in symbolic_execution.rs for consistency instead of checking for a prefix i though this is a non-blocking issue.

Feel free to merge or fix and merge.

[wu-benjamin]

Just a note that we can probably support i128 in the future by creating the Z3::ast::Int with the from_str function.