This commit improves the formalization of how we handle names.
The main changes are:
* Now `_` is a reserved prefix and all the generated names start with
`_`.
* The model verification routine now checks that `CustomName`s in the
global scope do not collide with any local namespace (e.g., fields of
a `StructType`).
* We changed the prefix `prefix_` to `unreserved_` to better convey the
fact that the prefix has been introduce to use an non-reserved name.
Ivan Krysak