Declare typedef's for public types

[ghost]

It is common practice to declare typedef's for public types. We should do the same, so that users don't need to type the struct keyword all the time.

Likewise, it is a common style to use CamelCase to name such types. We should probably follow this style too. Additionally, we should prefix all public types with "idl", e.g. "IdlUser" instead of "struct user".

[ChristianSch]

Typedefs seem to be bound to some controversies. The prefixes are quite important, I agree.