Eliminate the concept of imbuing messages in favor of the simpler concept of setting a cap table directly on MessageReader / getting one from MessageBuilder.  This eliminates capability-context entirely.  This was made possible by the earlier change which moved capabilities to a separate table rather than storing CapDescriptors inline, but I didn't realize it at the time.

Spin off libraries kj-async and capnp-rpc, so that people who don't want them can avoid their large footprint.

Revamp generated interface to Object fields.  Now there is an ObjectPointer::{Reader,Builder}.  This will simplify the dynamic API (next change) and also makes it easier to delegate decisions about the object type to a function that doesn't know about the context where the object lives (i.e. by passing an ObjectPointer::Builder).

The Fedora bug caused the DT_NEEDED for libpthread to be omitted, and
the missing -Wl before no-undefined prevented binutils from detecting it.

-export-dynamic is unnecessary for libraries, so this removes it, too.

Fix bug where only the first import made it into the import table.  Can't believe this wasn't tested.  Sigh.

Fix linking problems that only come up when using Ubuntu's libtool to build a dist and then installing it on OSX.  libkj.la was not listed in libcapnpc's deps.  Also put the pthread libs last since that's more correct.

Split libcapnp into three libraries:  libkj, libcapnp, and libcapnpc.  The third contains the schema parser.  Unfortunately it is rather enormous so we should give people the option to not link it.

Switch to the new compiler.  All tests pass\!  Need to submit bootstrap copies of the generated code used by the compiler itself.  These bootstrap copies are still built by the old compiler at the moment.

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.

Update Makefile.am to build new compiler.  Not bootstrapping yet, of course, since the C++ output plugin is not written.

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.