Fix invalid variable names

[ameliatastic]

Rust has some reserved keywords/names that are valid variable names in Python, such as _ (single underscore, used to throw away values) and match. If you use one of these as a variable name in a Seahorse program, you'll usually get a very weird rustfmt error.

# For example, this should not be allowed.
match = filter(..., array)

Ideally, Seahorse should raise an error when one of these names are encountered, and guide the user to choose a different name.

[Saksham093]

I'm working on this issue, I need more information.

[kubanemil]

Hey, can I handle this problem? Or does it no longer matter?