Add functions to (de)compress data blocks in one go

[rhpvorderman]

These functions would

• Be as simple as possible
• Create a buffer with pre-determined size given by the user.
• Return OverflowError when the buffer is full.
• Escape the GIL as much as possible.
• Use isal_stateless functions.

The functions can be used by downstream applications for bgzip (de)compression in a threaded manner. See #219 .

This can be implemented in igzip_lib and use the igzip function names.