* `basic_parser` now uses pure virtual member functions instead
of the curiously recurring template pattern.
Actions Required:
* Change uses of the `basic_parser` type to omit the `Derived`
template parameter
* Classes derived from `basic_parser` no longer need to friend
the base.
* Virtual functions in the derived class may be marked `override`.
This fixes a problem where an assert is generated or an
error is ignored when an invalid deflate stream is produced
after appending the final empty deflate block.
