This patch gets everything to the point of building the library, but it
doesn't run yet since I have missing symbols.
Unlike previous compatibility libraries and the concurrency
compatibility library, I'm organizing the headers a bit more. This is
because we're merging the two libraries into one. They share some common
header names, and while I could rename them for namespacing purposes,
it's easier to just use a directory structure for this.
The `include/Runtime` and corresponding `Runtime/` directories are for
backdeployed changes to the stdlib itself.
The `include/Concurrency` and corresponding `Concurrency/` directories
are for backdeployed changes to the concurrency runtimes.
