alignment with WATM v1 spec

WATM v1 spec is getting close to be finalized and once merged, will be replacing former WATM specifications 
(`v0` and former `v1`-preview will be removed). 

This repository should start implementing corresponding changes to support latest WATM v1 specification once it is finalized. 

See https://github.com/refraction-networking/watm/pull/16 for updated v1 spec. 