Code reusability

[tim-hoffman]

There are several simple functions in LlzkCodegen that only rely on the contained LlzkContext and could be commonly needed by other consumers of the llzk-rs repo:

• int_type()
• bool_type()
• index_type()
• felt_type()
• struct_type()
• index_attr()
• affine_map_attr()
• new_bool_const_op()
• new_int_const_op()
• new_index_const_op()
• new_felt_const_op() (or a more general version)
• location_unknown()
• new_nondet_at_location()