Flag to control how deeply macros are expanded

[dtolnay]

For example, avoid expanding try! and other macros inside the output of proc macros.

cc @djc

[dpc]

I'd like to request this to be an arbitrary level of macro expansion. I am particularly interested in 0.

For cargo-crev I had an idea that cargo-expand could be used for a one-file view of the source code for the purpose of quick code review. I'm mostly interested in mod file conversion to mod { ... }

[dtolnay]

I posted a relevant request for implementation for 0 levels: dtolnay/request-for-implementation#6.