style: Generate StyleTimingFunction and drop ns_timing_function.rs.

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,
(https://github.com/rust-lang-nursery/rust-bindgen/issues/1429)
So we have to hide StyleTimingFunction to avoid the compilation errors.

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

components/style/values/computed/easing.rs

@@ -5,7 +5,10 @@
 //! Computed types for CSS Easing functions.
 
 use values::computed::{Integer, Number};
-use values::generics::easing::TimingFunction as GenericTimingFunction;
+use values::generics::easing;
 
 /// A computed timing function.
-pub type TimingFunction = GenericTimingFunction<Integer, Number>;
+pub type ComputedTimingFunction = easing::TimingFunction<Integer, Number>;
+
+/// An alias of the computed timing function.
+pub type TimingFunction = ComputedTimingFunction;