Add deprecations and new methods to prep for IO rewrite

[cpubot]

In prep for #213, this PR deprecates methods that will be removed in next breaking release (0.5), and adds new methods that will be included in #213 to make migration possible before switching to 0.5.

Deprecated methods

• fill_buf: use copy_into_slice if you ultimately intend to copy the slice, or take_scoped for a call-scoped borrow. Be advised that take_scoped may not be implemented for all readers.
• fill_exact: use copy_into_slice if you ultimately intend to copy the slice, or take_scoped for a call-scoped borrow. Be advised that take_scoped may not be implemented for all readers.
• fill_array: use take_array.
• copy_into_array: use take_array or copy_into_slice.
• borrow_exact: use take_borrowed.
• borrow_exact_mut: use take_borrowed_mut.
• consume_unchecked - use one of the take_* methods, which automatically advance the reader.
• consume - use one of the take_* methods, which automatically advance the reader.
• peek - use take_byte.

New methods

• take_byte: Return the next byte and advance by 1.
• take_borrowed: Replaces borrow_exact.
• take_borrowed_mut: Replaces borrow_exact_mut.
• take_scoped: Return a call-site scoped immutable slice. The returned slice is tied to the borrow of self, meaning it is only valid while the current borrow of the reader is alive. Implementations are free to return slices backed by internal buffers or transient windows into the underlying source. This method is intended to be optional like take_borrowed and take_borrowed_mut, and return an error by default.

[kskalski]

Looks good, I would just use the opportunity to improve the comments for borrow functions slightly