Split cxx crate into runtime and build.rs components

[maurer]

Right now, the cxx crate contains both runtime components (e.g. cxx::unique_ptr) and build time components (e.g. cxx::gen, cxx::Build).

This is negative because:

• For cross compilation builds, this requires that what should be build-only dependencies be supported on the target.
• This builds more functionality into the target binary than needed. If cxx is linked statically, there's a good chance most of this will be removed, but it will bloat in cases where it's linked dynamically.
• For homogenous build systems (bazel, buck, etc.) this requires them to pull in the cc and link_cplusplus crates to their source tree, which will never actually be used.

The two ways I see forwards here are:

• Split it using features. Users of homogenous build system, or who cared about binary size, etc. would be expected to disable a default feature to get a support-code-only version of the crate.
• Split it into separate crates, e.g. cxx-support and cxx-build, possibly keeping the main cxx crate as a wrapper that re-exports both.

Opinions?

[dtolnay]

We should split the crate. I'll try to move the build-only stuff to a cxx-codegen crate and keep only the runtime components in cxx.

[maurer]

Do you want to handle that yourself, or should I send up a PR? I had it temporarily split with a feature to get it working in my context.

[dtolnay]

I will take care of this one since I'll probably be picky about the directory layout. It sounds like you have a workaround for now? If so, I'll aim to get this done and published early next week.

[dtolnay]

Fixed in 0.3.0.