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.

Implement 'Object' field type, which can be any pointer type.  The caller specifies the type when calling the accessors.

Detect and disallow List(Inline(Bool, n)) as it is not currently implemented correctly (and may be difficult to implement correctly-yet-efficiencly).

Correctly handle inline fields that aren't present in the parent struct (i.e. because we're reading an older version of the struct that lacked those fields).

ignore /compiler/dist from cabal install

Improve unions.  Members of a union are now scoped within the union, although they share ordinal space with the parent struct.