Remove Strings from the flat absy

[Schaeff]

The current core of the compiler uses string substitution to flatten constructs. This has a considerable cost especially during function call resolution.

Introduce FlatVariable which is cheap and Copy, and keep track of bindings between non-flattened variable names and FlatVariables in a bijection table.

Other changes introduced:

• Get rid of substring-based witness generation (x_b5 being interpreted as the 6th bit or x at run time) and instead use explicit Directives
• Remove loops from substitution calls: we only need to know the latest FlatVariable for a given Variable, not each intermediate one

To merge after #130

[Schaeff]

jenkins-test