Idea: cargo edit lib

Hi there!

First of all: Thanks for the project. It is great!

I've been working on [crossgen](https://github.com/yoshuawuyts/crossgen) to get support to build cross platform libs using Travis and Appveyor.

Sadly, I've come to face an [issue with cargo](https://github.com/rust-lang/cargo/issues/6160) where it is not possible to override the lib type through the command line.

One of the ideas I had was to be able to execute `cargo edit lib --crate-type=staticlib` and let it modify the manifest right before building. This would be added to the scripts, and would allow specifying only a single type of crate per target, without producing the `rlib` artifacts as well.

This would be very useful for C libs, but also WASM, as it needs to output a small library and it can't have the optimizations passed in if the manifest still have the `lib.crate-type` not overriden. ([See](https://github.com/rust-lang/rust/issues/51009))

What would you think of having this on `cargo-edit`? Or would you rather have this as a separate project?

Start by reviewing the cargo-edit CLI entry point for the proposed `cargo edit lib` command and how it currently handles Cargo manifests. Clarify whether the change belongs in cargo-edit or a separate project, then define the manifest update and build behavior needed for static libraries and WASM before implementation.

Written by the indexing model from the issue text.