Rename `Config` type

[brson]

This is not all that descriptive a name, though we're not sure what a better name is. I suggest Builder. If nothing better comes up, pick that and we'll bikeshed it on the PR.

cc @alexcrichton @sfackler @dtolnay @Kimundi @BurntSushi bikeshedding.

[opilar]

+1 for Builder

[dtolnay]

I am not a fan of Builder because it makes me expect a build() method that gives me back some object that I used it to build.

Some alternatives to explore:

• gcc::Orchestrator⸻https://en.wikipedia.org/wiki/Orchestration_(computing)
• gcc::Invocation⸻like Clang's clang::CompilerInvocation
• gcc::Build⸻in the noun sense like "build tool", "build automation"

[alexcrichton]

Build sounds pretty reasonable to me, becuase that's what's happening here anyway!