Support receiving current exe path as argument

[davidkna]

If an executable was renamed prior to calling self_delete (e.g. to allow external package managers to update it),
std::env::current_exe may return the pre-renamed path location, notably on Windows.

This PR adds a self_delete_at function that receives the expected exe location as an argument instead of attempting to infer it.
I only added a variant of self_delete, but if necessary I can also add _at variants for other functions.

[mitsuhiko]

Can you clarify why this is needed? In which situation does an external system rename the exe and how does one use this?

[davidkna]

I wanted to use this feature for topgrade-rs/topgrade#797. The topgrade executable has a feature to rename itself to allow package managers that it runs to update its main executable, and I wanted to use this function for cleanup in that case.