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

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

Update implemented pointer format to match docs:  Don't store field count in struct pointers because it isn't needed.  Instead extend data and pointer section sizes to 16 bits, which should be enough for anyone.  Also, store offsets from the end of the pointer rather than the beginning, because this makes it possible for them to be zero, which compresses better.  In particular, the root reference will always have a zero offset, so this will shave a byte off of very small packed messages.

Make enum syntax match docs, make enum lists work correctly, add enum tests, enforce maximum ordinals on enum values and interface methods, change the maximum field ordinal to 16-bit to match docs (now all ordinal limits match).

Basic code generation starting to work.  My machine just did something weird so I'm submitting and pushing in case of imminant hardware failure...