This commit adds support for recognizing "fingerprints" of upstream
dependencies. When a dependency's fingerprint change, it must be rebuilt.
Currently the fingerprint unconditionally includes the version of the compiler
you're using as well as a specialized version depending on the type of source
you're compiling from:
- git sources return their fingerprint as the current SHA. This will
disregard any local changes.
- path sources return their fingerprint as the maximum mtime of any file found
at the location. This is a little too coarse and may rebuild packages too
often (due to sub-packages), but this should get the job done for now.
When executing `cargo compile`, dependencies are not rebuilt if their
fingerprint remained constant.
This commit enables support for custom precompilation commands to be triggered
before a package builds via rustc. The current interface is to have
`build = "foo"` in the `[project]` section of Cargo.toml and cargo will just
execute the exact command given.
At the moment, the rustc exec for the root project inherits the stdout
and stderr FDs (so that warnings and errors flow through), but
output from dependencies is only emitted if the compilation fails to
avoid warning noise from dependencies.
At the same time, we started adding a generic error handling concept to
Cargo.
The idea is that every Result gets converted to a CargoError, which
includes all the information that Cargo needs to print out a friendly
human error message and exit gracefully.
