Better names mangling

[OleksandrKvl]

Prior to this change, all names were mangled in a way like message_N, type_N and so on. From compiler error messages it was hard to understand the problematic schema entity. With this PR:

• mangled names follow <original_name>_N pattern
• group entries have names like <group_name>_entry
• for public types and messages mangling is not used at all whenever possible

With the above changes it's now much easier to figure out the mapping between schema entities and implementation types.