Pull TextBlob out of capnpc-capnp and into KJ as kj::StringTree.  Use it in the stringification code to make that code cleaner and avoid using iostreams.

Add utility code for really nice command-line option parsing and start using it in the compiler (still WIP).

Implement mutex and once-init synchronization primitives.  Wrap pthreads for now, but plan on raw futexes later.

Enable some useful Clang warnings, fix some bugs it detected.  Especially, eliminate all dynamic initializers.

Mark every destructor "noexcept(false)" unless it is very clearly noexcept.  The change in C++11 making destructors default-noexcept was a mistake.