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
this makes sure the checks run before typeck (which might use the constant or const
function to calculate an array length) and gives prettier error messages in case of for
loops and such (since they aren't expanded yet).
