Warnings as errors

[davidiw]

Please give the option to make warnings as errors, especially for framework builds. It ensures that we don't let less than proper code hit our framework and disrupt our builders' experiences.

[brmataptos]

@davidiw how would you like this option to be controlled? E.g., an environment variable, command-line flag, Move.toml feature, special-casing stdlib/aptos, or what?