Start migrating upb_msglayout to be suitable for generated code.

This involves:

- remove upb_msglayout -> upb_msgfactory dependency.
- remove upb_msglayout -> upb_msgdef dependency (in progress).
- make upb_msglayout use a representation that can be
  statically initialized by generated code.

The goal here is that upb_msglayout becomes a kind of "descriptor
lite": it contains enough data to parser and serialize protobufs
and manipulate a upb_msg in memory, while being far smaller and
simpler than a full descriptor.  It also does not include field
names, which can be a benefit for applications that do not want
to leak field names.

Generated code can then create a upb_msglayout, and do most things
without ever needing to construct full descriptors/defs if they
don't want to.

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