Add mechanism for catching exceptions that has limited use even with -fno-exceptions, as well as a mechanism to detect when a destructor is called during unwind and to avoid throwing in these situations.

totalSizeInWords() method on generated struct readers/builders returns total size of the struct, not counting far pointer overhead.  Useful for allocating space for a flat copy.  Also a couple misc tweaks.

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