Blow away all directories/files on each build

And aftewards selectively move them back into place if they're fresh. This
prevents stale files from showing up from old builds.

This currently breaks anyone build `build=` scripts, the fix is coming in the
next commit.

Closes #205
House all native build output in a per-package dir

In order to ensure there are no stale artifacts as part of a build, this commit
houses all output of native build commands in their own directories. Each
directory is on a per-package basis, and the output is preserved if the package
is fresh or discarded if it is not.

This does not remove the DEPS_DIR environment variable, it just wires it to the
same value as OUT_DIR.
auto merge of #216 : alexcrichton/cargo/issue-205, r=wycats

As discovered in #205, cargo wasn't properly ensuring a fresh build state for new builds. As commented in #205, this is an easy way to have a non deterministic build which is one of cargo's major goals to avoid. These commits rectify the situation.

All build output is now "obliterated" by moving to a location unknown to the compiler as soon as a build starts. All fresh output is moved back into place automatically, and all dirty output will only have known artifacts available to it.

The major consequence of this strategy is that the location of the output of the `build` command must also change. Instead of spraying into a shared directory, build commands are now contained to their own build directory which is entirely removed on rebuild or entirely preserved on a fresh build. There's more documentation in the new `layout.rs` about the build directory structure.

In general this should not have much impact as a user-facing change.