1.0.0-beta.1

Added

• Marker::allocate_slice(), Marker::allocate_slice_clone(), and
  Marker::allocate_slice_copy() for creating slice allocations.
• Allocation::concat() and Allocation::concat_unchecked() for
  concatenating two adjacent allocations into a single slice allocation.
• MarkerFront::append{,_clone,_copy}() and
  MarkerBack::prepend{,_clone,_copy}() for extending existing allocations at
  the end of their respective stacks with new data. This functionality is also
  available through the Marker trait using the extend{,_clone,_copy}()
  methods (appending is always performed by front markers, and prepending is
  always performed by back markers).
• IntoSliceAllocation trait for safely coercing Allocation instances into
  allocations of slices. This is also used to determine what types can be used
  for allocation concatenation (concat() and concat_unchecked() allocation
  methods) and extension (append*() and prepend*() marker methods).
• SliceLike trait for performing conversions between DSTs that essentially
  wrap some primitive slice type and the underlying slice type (e.g. between
  str and [u8]), and ConcatenateSlice trait for marking SliceLike
  types that can be safely concatenated without any additional verification
  needed. This is used to allow for general use of such DSTs in allocations.
• Various unit tests for edge cases (e.g. allocation extension safety, ZST
  allocation support).

Changed

• Replaced the simple Error enum with an Error struct that provides the
  error category (an ErrorKind enum variant based on the old Error enum)
  and any values whose ownership was intended to be passed to the callee. This
  allows the caller to reuse such values if an operation fails instead of
  simply throwing them away.
• Replaced Marker::concat() with Marker::concat_slices_clone() and
  Marker::concat_slices_copy(), which both work on general slices.
• Use NonNull<T> for internal storage of Allocation pointers, allowing for
  compiler optimizations such as reducing Option<Allocation<'marker, T>>
  size to that of a single pointer.