support for null-restricted and nullable types

[Ladicek]

On the API level, this likely means just adding a method Type.nullness() which returns an enum with 3 values (UNKNOWN, NULLABLE, NULL_RESTRICTED) . However, this also involves changing the serialization format.

Per https://openjdk.org/jeps/8303099:

In Java syntax, a nullness marker is used to indicate this property.

The type Foo! is null-restricted: the value set excludes null.

The type Foo? is nullable: the value set deliberately includes null.

By default, the nullness of the type Foo is unspecified: a null may occur, but we don't know whether its presence is deliberate.

Signature attributes have an updated grammar to allow ! and ? in types, as appropriate. Nullness is not encoded in method and field descriptors.

However, to prevent pollution of fields, a new NullRestricted attribute allows a field to indicate that it does not allow null values.

This suggests that maybe we should add FieldInfo.isNullRestricted(), but let's leave that as an open question for now.

[Ladicek]

I'm not targeting this to any particular release, but I'm fairly sure this isn't needed any time soon.