Tracking Issue: Value References

[kellerkindt]

ValueReferences are the constants of ASN.1. With this PR constant definitions in ASN.1 are parsed.

• Parse and resolve basic Value References (Integers) Parse and resolve ValueReferences #49
• Add constant values to rust (75f235b)
• Support ENUMERATED values (0c1cd81)
• Properly parse hex values and other non-integer VRs
• an (yet) unsupported value reference literal of type 'TypeReference("...", None)' was discovered: "0" (simple values wrapped in tuples are not supported yet)

PR #49 changes public API and thus is a breaking change in this regard. A version bump to 0.3.0 is therefore required by the semantic versioning guidelines No. 4: "Major version zero (0.y.z) is for initial development. Anything MAY change at any time. The public API SHOULD NOT be considered stable."

This resolves #45