fix typos in error message and comment

Add zero arg constructor and `getType` to DynamicValue::Pipeline

Make Schema default constructors produce consistent, non-crashy values.  This improves the dynamic interface's robustness, particularly when exceptions are disabled.

Refactor capability code using fork.  Still too much refcounting, though.  Maybe this calls for a different design for pipelining...

Using promises for references to represent each branch of a fork doesn't work, because the fork hub will be deleted when the last branch resolves, before any continuation can run.  Instead, let's just institutionalize the concept of addRef() for owned pointers.

Interface code generation.  Also update method grammar to allow multiple returns, and allow specifying request/response structs instead of parameter lists.

capnp plugin: missing comment for union tag on groups.