First, we generate StyleComputedTimingFunction by cbindgen from Rust, and use
it in nsTimingFunction, so we could copy it directly without handling
the different memory layout. However, we have to rewrite the
nsTimingFunction and mozilla::ComputedTimingFunction for this.

Second, the rust-bindgen seems cannot generate the correct generic members
from complex C++ templates, especially for the nested template struct,
(rust-lang/rust-bindgen#1429)
So we have to hide StyleTimingFunction to avoid the compilation errors.

Differential Revision: https://phabricator.services.mozilla.com/D9313