In-place mocking

Temporarily overwrite the memory that stores all information about a function to be mocked (formals, body, environment, attributes). Now, also mocking imported functions works as expected.

Fixes #171.

Doesn't include #175 anymore.
Underneath, [`R_compute_identical`](https://github.com/wch/r-source/blob/653ed5777c551adddee1ad453480c1cb14deee57/src/main/identical.c#L85) is called. This function seems to check for deep identity of the two `SEXP` passed, referential identity is only a shortcut (line 89).