Merge #122: Add transparent representation for u5

02a5686 Add transparent representation for u5 (Nikolai Golub)

Pull request description:

  As mentioned in #19 having transparent representation for u5 opens many optimization.

  According to change log in Rust repo, this feature was [stabilized in 1.28](https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1280-2018-08-02)

ACKs for top commit:
  apoelstra:
    ACK 02a5686

Tree-SHA512: d6ad67cc9740279bb969d73b0a2ad2e7319ac7272c91c63af8751e77571758b85edce87abc2aa7ef1567afe68c6bf7a4c07a88656035533137d98d1a8e30f376

src/primitives/gf32.rs

@@ -65,6 +65,7 @@ const CHARS_INV: [i8; 128] = [
 
 /// An element in GF(32), the finite field containing elements `[0,31]` inclusive.
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[repr(transparent)]
 pub struct Fe32(pub(crate) u8);
 
 impl Fe32 {