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

Revamp dynamic struct getters/setters to not distinguish unions.  Add converience versions that take string names.

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

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.

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.

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