ignore /compiler/dist from cabal install

Improve unions.  Members of a union are now scoped within the union, although they share ordinal space with the parent struct.

Get rid of redundant ErrorReporter that goes in ReaderOptions.  Registering an ExceptionCallback is a better approach.

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