Move away from "whitelist"/"blacklist" terminology

[Manishearth]

Bindgen currently uses "whitelist" and "blacklist" terminology in its flags. The Rust project and the industry in general have been slowly moving off of these due to their negative implications (rust-lang/rust#74150, rust-lang/rust-clippy#5896), perhaps we should do the same here? I think we can keep an alias for backwards compatibility, but document allowlist/denylist instead.

[emilio]

Dupe of #1812, there's a patch about to land in #1990.

[Manishearth]

wow, amazing timing