add visibility information for fields and methods

[mgudemann]

membert has Booleans is_public, is_private and is_protected. This was used, but not set in fieldt and set but not used in methodt.
This also adds ID_default when no other flag is set, in Java this allows visibility in the package, but not subclasses outside the package (cf. https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html)

[tautschnig]

Minor suggestions for improvements.

[tautschnig]

I think this blank line is safe to stay ;-)

[mgudemann]

the line is back :-) this was an artifact from squashing some commits

[tautschnig]

Shouldn't this be using the same set_access API? (member_type will thus need to be converted to a more specific type than typet.)

[mgudemann]

the problem is that set_access is defined for struct_union_typet and therefore is only really usable for fields here. I had thought about using a special java_typet derived from typet which could have the set_access, too. Would you prefer that?

[tautschnig]

What I would like to avoid is having typet objects floating around that have ID_access set without anything ever making use of that. Notably, would equality comparison succeed on two typet with ID_access set to different values -- likely not. Thus such information should either be communicated via "#access" ("ID_C_access") or with objects of proper type and API. Maybe it takes introducing java_typet, but then this needs to propagate to, e.g., base_type_eq.