deny warnings

[burrbull]

warning: lint `plugin_as_library` has been removed: `plugins have been deprecated and retired`
 --> src/stm32f100/mod.rs:6:9
  |
6 | #![deny(plugin_as_library)]
  |         ^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(renamed_and_removed_lints)]` on by default

warning: lint `safe_extern_statics` has been removed: `converted into hard error, see https://github.com/rust-lang/rust/issues/36247`
 --> src/stm32f100/mod.rs:8:9
  |
8 | #![deny(safe_extern_statics)]
  |         ^^^^^^^^^^^^^^^^^^^

warning: unknown lint: `unions_with_drop_fields`
  --> src/stm32f100/mod.rs:10:9
   |
10 | #![deny(unions_with_drop_fields)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unknown_lints)]` on by default
``