Compiler actually running plugins!  Hooked it up to capnpc-capnp (the loopback plugin).  Working on getting the output identical to the old capnpc...  still a few bugs left.

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

typos

Futex-based mutex and once-init implementations for Linux.  These are measurably faster -- and much smaller -- than the pthread-based implementations.

Allow SchemaLoader to use a callback to lazily load schema nodes, including dependencies of loaded nodes.

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

This should have been in the last commit but apparently I botched the --amend and then pushed without realizing.

Complete C++ .capnp parser.  Also, add a pretty-printing function for arbitrary Cap'n Proto objects which is like the existing stringification but uses whitespace (newlines an indentation) to make the structures more readable.