The facet of a stage is rarely relevant when running a tool or building
something, it's all a question of what stage the *compiler* is built in. We've
already got a nice handy `Compiler` structure to carry this information, so
let's use it!
This refactors the signature of the `Build::cargo` function two ways:
1. The `stage` argument is removed, this was just duplicated with the `compiler`
argument's stage field.
2. The `target` argument is now required. This was a bug where if the `--target`
flag isn't passed then the snapshot stage0 compiler is always used, so we
won't pick up any changes.
Much of the other changes in this commit are just propagating these decisions
outwards. For example many of the `Step` variants no longer have a stage
argument as they're baked into the compiler.
This commit is the start of a series of commits which start to replace the
makefiles with a Cargo-based build system. The aim is not to remove the
makefiles entirely just yet but rather just replace the portions that invoke the
compiler to do the bootstrap. This commit specifically adds enough support to
perform the bootstrap (and all the cross compilation within) along with
generating documentation.
More commits will follow up in this series to actually wire up the makefiles to
call this build system, so stay tuned!
