Use TryInto to avoid unsafe.

Previously, to_fields and to_fields_le used unsafe to convert a
&[u8] into a &[u8; 8]. Now that we're only supporting Rust
versions where TryInto is stable, we can use try_into().unwrap()
instead, making uuid entirely safe Rust.

In release mode, the compiler detects that the slice will always
be the correct size, so try_into can never fail. Thus, the unwrap
is optimized out and we end up with the exact same assembly
as the unsafe block.

Godbolt output showing the resulting assembly:
https://godbolt.org/z/nWxT6W

Closes #488.

src/lib.rs

@@ -198,7 +198,7 @@ mod v5;
 #[cfg(all(windows, feature = "winapi"))]
 mod winapi_support;
 
-use crate::std::{fmt, str};
+use crate::std::{fmt, str, convert::TryInto};
 
 pub use crate::error::Error;
 
@@ -390,8 +390,7 @@ impl Uuid {
         let d3 =
             u16::from(self.as_bytes()[6]) << 8 | u16::from(self.as_bytes()[7]);
 
-        let d4: &[u8; 8] =
-            unsafe { &*(self.as_bytes()[8..16].as_ptr() as *const [u8; 8]) };
+        let d4: &[u8; 8] = self.as_bytes()[8..16].try_into().unwrap();
         (d1, d2, d3, d4)
     }
 
@@ -431,8 +430,7 @@ impl Uuid {
         let d3 =
             u16::from(self.as_bytes()[6]) | u16::from(self.as_bytes()[7]) << 8;
 
-        let d4: &[u8; 8] =
-            unsafe { &*(self.as_bytes()[8..16].as_ptr() as *const [u8; 8]) };
+        let d4: &[u8; 8] = self.as_bytes()[8..16].try_into().unwrap();
         (d1, d2, d3, d4)
     }