FIX: Adjust (mostly remove) inline directives

Using the following principles:

- Trivial methods (empty and simple constants) can be inline(always)
- Generic methods don't need any inlining directive at all.
  ..with the exception of Deref which just seems to be extra important.
- Non-generic items use #[inline] to enable inlining at the
  compiler's discretion.

src/array.rs

@@ -87,10 +87,8 @@ macro_rules! fix_array_impl {
             type Index = $index_type;
             const CAPACITY: usize = $len;
             #[doc(hidden)]
-            #[inline]
             fn as_slice(&self) -> &[Self::Item] { self }
             #[doc(hidden)]
-            #[inline]
             fn as_mut_slice(&mut self) -> &mut [Self::Item] { self }
         }
     )