antlr4_7: init at 4.7

Includes a more recent version of antlr to nixpkgs. Previous
versions exist already, but version 4 brings many changes
to the generated code and runtime targets.

The install location has been changed from previous versions
of antlr to make use of the set-java-classpath hook, which
is required to make use of both the runtime and the binary.

Also includes the testing rig as a script to allow graphical
inspection of parse trees.