Name rustbuild instead of saying 'the bootstrap binary'

[jyn514]

r? @Mark-Simulacrum - is this reasonable? It's consistent with the A-rustbuild tag on rust-lang/rust.

[jyn514]

Separated out from #807

[Mark-Simulacrum]

I think we should wait to land this until we've renamed it upstream.

[mark-i-m]

Is there a particular PR on rust-lang/rust that this is blocked on?

[jyn514]

Not that I'm aware of.

@Mark-Simulacrum are you sure it makes sense to rename the whole directory? In my mind 'rustbuild' is just the rust portion while 'bootstrap' is both python and rust put together.

[Mark-Simulacrum]

I've never separated the two, personally, but I also don't care much either way.

[mark-i-m]

What's the status of this?

[jyn514]

I don't think this is a useful change; there's no point in adding new terminology if it's not going to be used consistently.