Defer field access in ComputeMethodIndex subjects

This is allows a possible future change to the generation order so
that subjects can contribute to the ctor.  Made these subjects and
their coders internal since they are never used outside of the
ComputeMethodIndexMixin.
Safe proxies check method exists on real subject

Applies to duck proxies now, but will include any proxy type that
implements IMethodExistsProxyMeta.