polyval: unify state for intrinsics backends (#299)

Instead of having each intrinsics backend (i.e. `avx2`, `neon`) define
its own `State`, this defines a single `ExpandedKey` struct that can be
shared between both intrinsics backends and a fallback software
implementation, all defined in terms of `FieldElement`, as well as a
common `State` struct.

The `State` struct replacing the old union becomes a good place to put
`InitToken`, so we don't need to have a dummy `InitToken` for the soft
backend.

The `soft` backend now is only compiled in if we're not using intrinsics
whatsoever, and uses a much more compact implementation which stores `h`
directly instead of a powers-of-H expansion, intended for low-power
targets where resource conservation is more important than performance.

Author: tarcieri

polyval/src/backend.rs

@@ -1,23 +1,13 @@
-mod soft;
-
 cpubits::cfg_if! {
-    if #[cfg(all(target_arch = "aarch64", not(polyval_backend = "soft")))] {
-        // PMULL/NEON backend for aarch64
-        mod autodetect;
-        mod neon;
-        pub(crate) use autodetect::State;
-        pub(crate) use neon::InitToken;
-    } else if #[cfg(all(
-        any(target_arch = "x86_64", target_arch = "x86"),
+    if #[cfg(all(
+        any(target_arch = "aarch64", target_arch = "x86_64", target_arch = "x86"),
         not(polyval_backend = "soft")
     ))] {
-        // CLMUL/AVX2 backend for x86/x86-64
-        mod autodetect;
-        mod avx2;
-        pub(crate) use autodetect::State;
-        pub(crate) use avx2::InitToken;
+        mod intrinsics;
+        pub(crate) use intrinsics::State;
     } else {
         // "soft" pure Rust portable fallback implementation for other targets
-        pub(crate) use soft::{State, InitToken};
+        mod soft;
+        pub(crate) use soft::State;
     }
 }