Review bounded_int_mul libfunc

[JulianGCalderon]

Review the bounded_int_mul libfunc, to ensure that all possible inputs are taken into account.

• What types can the libfunc arguments have?
• Is the offset of all bounded int values taken into account?
• Is the value of the return bounded int (if any) computed correctly, according to its offset?

We should:

• Document the signature the signature of the libfunc, including input and output values.
• Document the expected behavior of the libfunc.
• Add unit/integration tests for edge cases.