Add 16 bit buffers for all 16 bit types in ColorType. Include these as
supported types in DynamicImage. These 16 bit images are currently only
supported with u16 representation; changing this would require
specialization and macroing of color conversions.

Load 16 bit PNGs at full bit depth rather than squashing to 8 bit.

Note that tests will not pass until upgrading to a release of image-png
including image-rs/image-png#174.

Closes image-rs#560.
Closes image-rs#665.
Closes image-rs#940.

This ImageEncoder trait is currently minimal, but establishes a uniform
interface so that all formats have native endian encoding interfaces,
which is necessary for 16 bit per channel encoding.

Closes image-rs#1076.

This fixes 16bpc tests.

Per review comments, this assumes equal channel bit depths, which will
not be the case in the future.

Because the return type depends on the buffer's bits per channel, make
the return optional and provide separate methods for 8 and 16 bit
images.

Instead create a crate-local AsBytes wrapper trait. This trait is not
sealed as Primitive was not sealed, so that users can delegate it for
newtypes if needed.

Bytemuck is more feature constrained and builds on Rust 1.34.2.