Add benchmarks

[santiweight]

In order for us to refactor some current functionality (such as #7 or #55), it would be good to get an idea of how first the current code is! Otherwise we are shooting in the dark as to whether an optimisation is in fact an optimisation...

Benchmarks should cover:

• memory usage
• runtime efficiency

We should also do a pass to check whether we can improve strictness and whether there are any fields that can be UNPACKED (hint: there are)