Arrange for compiler to send to the code generator the complete contents of all files listed on the command line, all first-level dependencies of those files, and parent scopes of those dependencies, but nothing more.  This means it's sometimes possible to compile a schema file without parsing all transitive dependencies.  Also, in 'decode' mode, don't compile annotations, so that the files defining those annotations need not be opened at all.

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.

Replace most CAPNPROTO_ASSERTs with the new logging/error macros.  Also add a SYSCALL macro and use it.  Also add commentary.

Rename the 'Message' interfaces to Arena and make them internal.  Make a new, intuitive 'Message' interface for creating / reading messages.