rustpkg should put build output in a target-specific subdirectory

[catamorphism]

Right now it goes in the build/ subdirectory of the workspace, but it should be x86_64-apple-darwin/build/ or the like. Likewise for lib and bin.

[catamorphism]

I'm going to imitate what the Go package manager does: http://golang.org/cmd/go/#hdr-GOPATH_environment_variable

So build/ and lib/ will have target-specific subdirectories, but bin/ won't.