More systematic approach to building and testing libraries in /lib

[remysucre]

Right now one can only test /lib through DDlog tests, e.g. by using stack test --ta .... Adding separate tests for it would ease development.

[ryzhyk]

Expanding this issue to include a more systematic standard library design. So far we've added functions/modules to the standard library on-demand. We should adopt a more systematic approach, which involves implementing complete APIs for various supported data types, testing and benchmarking them ahead of time. The best way to do this is probably to model them after existing Rust standard libraries.