alignment with WATM v1 spec

[gaukas]

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 refraction-networking/watm#16 for updated v1 spec.