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.

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.