This commit removes the `-D warnings` flag being passed through the makefiles to
all crates to instead be a crate attribute. We want these attributes always
applied for all our standard builds, and this is more amenable to Cargo-based
builds as well.

Note that all `deny(warnings)` attributes are gated with a `cfg(stage0)`
attribute currently to match the same semantics we have today

Wouldn't want to be able to link to this on stable Rust!

The deny(warnings) attribute is now enabled for tests so we need to weed out
these warnings as well.