File inclusion with an explicit destination path

[LukeMathWalker]

As discussed in #9, this PR changes the behaviour of the include attribute:

• #[assay(include = ["folder/my_file.rs"]) will copy the file into the root folder of the test working directory (i.e. file.rs);
• #[assay(include = [("folder/my_file.rs", "folder/my_renamed_file.rs"]) will instead use the specified destination path.

I have a few questions on the implementation that I've left as TODO comment in the code. I'd appreciate your input there.

Closes #9

[LukeMathWalker]

Thanks for the review! TODOs are gone, the generic issue has been solved and the panics have been polished. It should be ready for a second look 😁