Ensure API compatibility with older versions.
Prepare version 3.1.5.
Remove now-untrue warning on the parser.
[maven-release-plugin] prepare release protoparser-3.1.5
[maven-release-plugin] prepare for next development iteration
Switch naming from types to elements.

This makes more sense conceptually since we are matching parsing and representing the syntax tree, not a type graph.

Since this is a fundamental API change (with more to come) we also bump the version to be working towards a v4.