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

Dynamic API mostly done...  not tested.  This commit actually starts embedding the schema data in the generated code.

Restore ability to upgrade from List(Bool) to List(T) where T is a struct whose @0 field is of type Bool.  I previously disallowed this to reduce complexity, but it turned out to actually increase complexity.

Starting on dynamic message manipulation (manipluating messages without knowing the schema, or without compiling in the schema).

Implement code generator plugins.  The Cap'n Proto compiler, when given -o foo, invokes the binary capnp-foo and passes the schema, in Cap'n Proto format, to its stdin.  The binary is executed with the current working directory set to the desired output directory, so all it has to do is write the files.

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

Rename the 'Message' interfaces to Arena and make them internal.  Make a new, intuitive 'Message' interface for creating / reading messages.

Moar type-safety, rewriting WireFormat to avoid descriptors (incomplete), decided struct pointers should point at the beginning of the struct after all.