Non-finite internal branching

[YaZko]

We currently restrict internal branching to finite branching, indexed by Fin types.
This is a clear restriction, and an immediate blocking point in experimental applications investigated by @nchappe.

Two possibilities:

• Allow for indexation by either a Fin type, or by nat
• Allow for indexation by an arbitrary type. In this case, the indexes used by a given computation must become a parameter of the datatype, similar to E for external interactions.

Exposing statically the indexes used in the second solution is mandatory if any notion akin to interpretation of internal non-determinism is to be defined in the future (and it shall!).

[YaZko]

See the branch choice-gen by @nchappe which addresses this issue.

We plan on merging it to master eventually, but probably not before a first 1.0 without it is stable.