Rewrite cxx-build crate

[dtolnay]

Fixes #213. Fixes #88. Supersedes #214. Supersedes #253.

The major improvement is that we no longer care where Cargo's target dir is located. Instead of doing anything based on relative paths from the target dir, we do it only based on the paths the user passes to cxx_build::bridge, which we know are relative to the manifest dir because that's the current directory when a build script runs.

For an invocation like cxx_build::bridge("src/lib.rs") the resulting layout of the OUT_DIR would be:

$  exa -T target/debug/build/cxx-demo-1848f7a3fc5320f1/out/
target/debug/build/cxx-demo-1848f7a3fc5320f1/out
└── cxxbridge
   ├── cxx-demo
   │  └── src
   │     ├── lib.rs -> ./lib.rs.h
   │     ├── lib.rs.cc
   │     └── lib.rs.h
   └── rust
      └── cxx.h

and the include paths would be #include "cxx-demo/src/lib.rs.h", where cxx-demo is the crate name and src/lib.rs is the path that the crate's build script gave cxx_build.