Cleanup tests

[Nadrieril]

This PR:

• Merges hashmap and hashmap_main;
• Renames betree_main to betree;
• Adds a way to specify options to the test runner for crates, in crate/aeneas-test-options;
• Adds some tests we wanted to add.

Fixes #208, #153, #195, #191.

[sonmarcho]

This is great! I would like to talk about the betree before you merge however (and we can decide whether we do the change now or in a later PR).