Remove __m64 support

[vks]

All MMX intrinsics were removed from Rust: rust-lang/stdarch#823

This currently causes packed_simd to not compile (see for instance #1047). Here is the PR that fixes packed_simd: rust-lang/packed_simd#292
Once the new version of packed_simd is released, we will have to remove support for __m64 to make the tests pass.

[vks]

Also see rust-lang/packed_simd#298.

[vks]

This patch uses the fixed packed_simd version:

diff --git a/Cargo.toml b/Cargo.toml
index 40ea48204e..9284f3084f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -61,8 +61,8 @@ serde = { version = "1.0.103", features = ["derive"], optional = true }

 [dependencies.packed_simd]
 # NOTE: so far no version works reliably due to dependence on unstable features
-version = "0.3"
-# git = "https://github.com/rust-lang-nursery/packed_simd"
+package = "packed_simd_2"
+version = "0.3.4"
 optional = true
 features = ["into_bits"]

However, we still need to fix the following line:

   --> src/distributions/utils.rs:190:35
    |
190 |     wmul_impl_16! { u16x4, __m64, _mm_mulhi_pu16, _mm_mullo_pi16 }

__m64, _mm_mulhi_pu16, and _mm_mullo_pi16 no longer exist and were replaced.