Improve name resolution for top-level scope, structs and enums

[denismerigoux]

Right now, name resolution is done in two passes (in file src/catala/catala_surface/name_resolution.ml):

1. Look at scope, struct and enum declarations in order and create identifiers for them and their fields/cases/context
2. Look at scope uses and link the identifiers to the ones created in 1.

This is nice but lacks flexibility for scope, structs and enums. Especially, we want users to be able to declare their scope anywhere in the code, and not following a dependency order. For instance, if scope A has a subscope of type B, we don't want to force the user to declare B before A.

This problem can be solved by doing a three-pass name resolution with the following passes:

1. Look at scope, struct and enum declarations and create identifiers for them
2. Look at fields/cases/context are create identifiers for them, also possibly linking to the scope/struct/enum identifiers of 1.
3. Look at scope uses and link the identifiers to the ones created in 1. and 2.