Spin off non-capnproto-specific stuff off into a separate library KJ (first pass).
Also add io.{h,c++} to KJ.
kj/common.h
Move Maybe into common.h and improve its interface.
tweaks
Split array and string stuff out of type-safety.h.
Improve heap allocation.
Improve String.
Move FixedArray and CappedArray to array.h.
Stringification for all.
Update Makefile.am