It allows `coverage::Key` to be `Send` & `Sync` - otherwise,
wrapping coverage data with a `spin::Mutex` will fail due to:
```
error[E0277]: `*const u8` cannot be sent between threads safely
--> src/coverage/mod.rs:49:23
|
49 | static COVERAGE_DATA: Mutex<FnvIndexMap<Key, u64, { 1024 * 1024 }>> =
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `*const u8` cannot be sent between threads safely
|
= help: within `IndexMap<Key, u64, hash32::BuildHasherDefault<hash32::fnv::Hasher>, 1048576>`, the trait `Send` is not implemented for `*const u8`
note: required because it appears within the type `StrBuffer`
```
[no changelog]
