Allow showing the expanded macros of only a given module / source file

[Boscop]

When working with many macros, looking at the expanded macro output of the whole crate doesn't scale.

There should be an option to only show the expanded macros from a given module / source file.

E.g. in one case I have 81733 lines in the expanded macro output from cargo rustc -- -Z unstable-options --pretty=expanded...

[dtolnay]

I should have reported this when dtolnay/cargo-expand#8 was filed, but 👍 from me and 👍👍👍👍👍👍👍👍👍👍👍👍 from 12 people over there.

[dtolnay]

I implemented a workaround in cargo-expand 0.4 for now by filtering the compiler's output through Syn. For example you can run:

expand contents of one module:

$ cargo expand punctuated::printing

expand a single item (struct, function, trait, etc):

$ cargo expand token::FatArrow