Variant: separate api for type conversion fields

As described in issue #175 we decided to provide two different APIs for
individual and shared field access. One does implicit conversions and
the other one throws exceptions if the type requested is not the same
the field was encoded in.

fixes #175.
yes, let's please not forget to make these calls by index as soon as it's possible to do so, so that the SANE way of accessing fields is not penalized by a double hash lookup!