Breaking changes to build_test coming

[davidmorgan]

Hi everyone,

I'm refactoring the various Reader implementations, and as part of that I'm making the test implementations closer to the real thing.

For example the in memory reader and writer classes are now merged so they always share state, rather than being allowed to be separate.

This leads to various simplifications, but also to removing some functionality that will require some tweaks to tests to continue using test_builder.

I don't want to cause anyone undue pain, so if anything turns out to be too annoying, I'm happy to work with you to ease the transition--just let me know.

Thanks!