Use correct `x = nil` instead of `x = any` to zero interfaces.

Fixes #383

Adds `zerocopy` tag that will allow byte slices to be unmarshaled with zero-copy, referencing the original slice.

AFAICT not possible for other types or when decoding.

Adds `//msgp:timezone utc` that will decode and unmarshal to UTC time.

Avoids nasty hacks, like setting `time.Local` or having to do reflection to go through structs.

Went for a new function, since it was a much cleaner solution than trying to convert after reading.

Package setof allows serializing sets map[T]struct{} as arrays.

Nil maps are preserved as a nil value on stream.

A deterministic (sorted) version is available, with slightly lower performance.