Downgrade crate-level lints to 'warn'

We recently fixed a compilation failure in some test code caused by a
use declaration that was ultimately unused. To this point we were under
the impression that there was a set of lints that were known to be
potentially changing in backwards incompatible ways while others would
not. Discovery of unused use declarations or symbols was always assumed
to fall into the latter category.
Because such a build breakage due to semantic change in what a lint can
detect by now is a repeated pattern (with dire consequences), this
change downgrades all 'deny'-style lints to warnings in order to prevent
similar problems in the future.

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