Conform all ASM functions to a module structure (xiph#1717)

This includes finding any ASM functions that are
scattered throughout the codebase and moving them
into the asm module, but also reorganizes
the asm module so that x86 is a module
containing submodules for each native module.
This reduces the number of cfg statements needed,
and will also make it cleaner to implement assembly
for other architectures moving forward.

Partially addresses xiph#1714

src/lib.rs

@@ -66,8 +66,6 @@ mod deblock;
 mod encoder;
 mod entropymode;
 mod lrf;
-#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
-mod lrf_simd;
 mod mc;
 mod me;
 mod metrics;