This refactors the drain_the_queue function to return a
tuple containing Result. That way, it's still not
possible to use ? or try! to handle errors,
but for readers of the function declaration it's
clearer now that the error actually indicates one.

Bonus: it makes the calling code of drain_the_queue simpler.

This commit adds an experimental flag to cargo, -Zwarn-unused-deps,
with the goal of getting it eventually stabilized and enabled by
default.

The lint builds upon the --json=unused-externs flag of rustc
that is being proposed for this purpose.
This commit makes cargo pass the flag if -Zwarn-unused-deps is enabled
to compilations of units it prints warnings for.

During compilation, code collects the unused dependencies
from all units, converts them into used dependencies,
continuously extending the record of used dependencies for
any type of DepKind: [dependencies], [dev-dependencies] and
[build-dependencies].

Once the compilation has ended, this record is used to
obtain those dependencies in a class that weren't used by any unit,
and warn about them.

The goal is to stabilize the flag and enable it by default
once the lint has become robust and the robustness is proven.

Then, cargo shall opportunistically warn about unused dependencies
when it has compiled (or loaded the unused externs from cache of)
all units that could possibly use dependencies of the kind.
Roughly, it's like this:
  * cargo always compiles build.rs
  * cargo check compiles all units that would use [dependencies]
  * cargo test --no-run --all-targets compiles all units that can use
    [dev-dependencies]... except for the benches
  * cargo check --all-targets compiles all units that can use
    [dev-dependencies]... except for the doctests

Specifying via the command line is not possible for now because cargo
currently has to pass -W via the command line, but once the lint
is set to warn by default, this won't be needed :).