blocks: Add the possibility for other blocks to vanish on build.

Refs #410.
blocks: Rename emerald block for uniqueness.

Fixes a test.

Refs #394.
beta/protocol: Building on top of the world is no longer lethal.

In addition, the top of the world was incorrectly hardcoded.
beta/protocol: Build the functionality of build_snow into the build hook.

Refs #410.
Remove build_snow plugin.

Fixes #410.