Standards.ASN1: Improved support for tagged types.

The decoder and MetaExplicit now have some minor knowledge about
ASN.1 tagged types.

Also adds the convenience symbols Standards.ASN1.Types.TaggedType[0-3].
Standards.ASN1.Types.MetaExplicit: Improved compatibility.

The objects generated via MetaExplicit now behave more like
the Compound objects they emulate.

Also fixes the argument order in Object()->get_combined_tag(),
which had the arguments swapped.

Fixes ASN.1 tests in the Standards testsuite.