project-wide tidying, style updates

[cpu]

It might be best to review this commit-by-commit. The total overall changeset should have no functional effect - it's exclusively stylistic changes.

• Removes explicit typing throughout, whenever the compiler can successfully infer the types on its own
• Fixes redundant prefixing throughout, whenever an item was already in-scope without the prefix
• Fixes a couple clippy findings that fell out of ^
• Applies a project-wide formatting pass, including within the ffi_panic_boundary! macro bodies (using the approach ctz used in https://github.com/rustls/rustls-ffi/pull/383 and that we're using in rustls-openssl-compat).
• Updates make format and make format-check to enforce rust formatting within the ffi_panic_boundary! macros

Resolves https://github.com/rustls/rustls-ffi/issues/31

[ctz]

Good stuff!

[cpu]

@jsha Would you like me to hold merging this until you've had a chance to review?

[jsha]

Nope, go ahead and merge, thanks!