This seems like a fairly important thing to have in the quickstart.

* Add "clearomitted" directive

Adds `//msgp:clearomitted` directive.

This will cause all `omitempty` and `omitzero` fields to be set to the zero value on Unmarshal and Decode, and the field wasn't present in the marshalled data.

This can be useful when de-serializing into reused objects, that can have values set for these, and we want to avoid clearing all fields, but also don't want existing fields to leak through.

Fields are tracked through a bit mask, and zeroed after the unmarshal loop, if no value has been written.

This does not affect marshaling.

`[][]byte` would assume that the byte slices had allownil set. This would cause an extra `if x == nil {x = make([]byte,0)}` to be inserted on elements.

Regression from #363

* Fix allownil on maps elements
* Also set for primitives.

Similar to #374 map `[]byte` elements would also inherit allownil unintentionally.