Feat: implement buses

[Leo-Besancon]

PR implementing #183, targetting upstream next (instead of massalabs#8 on massalabs's fork).

The goal of this PR is to make an initial implementation of buses, that can be declared:

buses {
    unit p,
    mult q,
}

Currently, only empty bus boundary constraints are handled (that are reduced to a 0 or 1 value depending on the bus type)

boundary_constraints {
    enf p.first = null;
    enf p.last = null;
}

Boundary constraints with public inputs depend on #143 and will be handled in a future PR.

integrity_constraints {
    p.add(a) when s1;
    p.rem(a, b) when 1 - s2;
}

What was done in this PR:

• Parse bus related expressions (AST)
• Handle semantic analysis and constant propagation for buses (AST)
• In a new MIR > MIR pass, before lowering to the AIR, expand bus integrity constraints and handle empty bus boundary constraints
• Add tests (in parser, Mir, Air, and Codegen) for new bus expressions
• Add documentation on buses

In future PR, we will introduce, in order of priority:

• Update the bus constraint handling (MIR/AIR > CodeGen)
  • In the AIR, add bus structs with an exhaustive description
  • Prepare expanding the constraints in CodeGen
• Handle public inputs as Bus constraints
  • Needs variable-length public inputs support, see Add support for variable-length public inputs #143
  • Needs to store the Bus constraint as a separate struct in the AIR (see above)
  • Needs Codegen update to generate the resulting constraint
• Remove aux and random value access in Front-End (see branch https://github.com/massalabs/air-script/tree/feat-implement-bus-remove-aux-rand, currently all tests pass except for Codegen)