Adds buffer based IO support.

[jonex]

This allows language wrapping libraries such as pyreadstat to use a memory buffer instead of a file on disk as source.

[camjw]

Hi, we're using Pyreadstat which depends on Readstat at Channel (https://usechannel.com) to let our users query SPSS files. We're very keen for this PR to be merged - having to save uploaded files to disk and then read them is much more annoying than it sounds. Our plan at the moment is to maintain a separate fork of Pyreadstat and apply the corresponding changes ourselves but would love it if this PR could be merged! Thanks for this @jonex !