rename nullable to optional everywhere

[fetsorn]

Right now msgpack (de)serializers have methods like "read_nullable_array", "write_nullable_array", "applyNullable". But the underlying implementations use option types, not nullable types.

The core difference between option types and nullable types is that option types support nesting (e.g. Maybe (Maybe String) ≠ Maybe String), while nullable types do not (e.g. String?? = String?). -- Wikipedia

Both assemblyscript and rust methods should be renamed to "read_optional_array", "write_optional_array", "applyOptional" etc.
Haven't done it in #936 because its scope did not include rust.