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.

Discarding version of char parsers.  Also, many() should return a count rather than an array if sub-parsers have empty results.