[WIP] @bobhy's Extern crates in doc tests

[pczarn]

Allows doc tests to freely reference external crates in use statements.
Avoids depending on unstable Cargo interfaces to determine dependencies by the
simple expedient of running a cargo build --verbose and observing how Cargo invoked the compiler.
Includes necessary doc changes.

Fixes #706 and #394.
Does not address #768.

[pczarn]

Original @bobhy 's code in #2503.

[pczarn]

@ehuss Would you take a look to see whether this is a valid design? The PR used to be hacky, and it still remains hacky, hopefully less so.

[pczarn]

@ehuss

Alternative design

• create a temporary directory
• put Cargo manifest into that directory
• use cargo locate-project --workspace and CARGO_WORKSPACE_DIR=path to use the original workspace for the temporary project
• generate code either as integration tests or as doc tests on modules (?)
• test the temporary project with cargo test [--doc] --target-dir=path/to/target