Refactor compiler binary into a multi-tool with sub-commands like 'compile' and 'decode'.  The latter dynamically decodes binary input to text based on a provided schema file.

Mass rename 'reference' -> 'pointer'.  Been meaning to do this for a while -- all the documentation talks about 'pointers', not 'references'.

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.