Spurious compiler warnings due to @Nullable annotations

[akirschbaum]

Recent Grappa versions annotate Var.get() with @nullable. While technically correct, this causes spurious warnings in parsers:

Rule r() {
    Var<String> v = new Var<>();
    return sequence('x', v.set("x"), 'y', f(v.get()));
}

In this code fragment the function f(String) cannot take a @nonnull parameter even if the actual value returned by v.get() is never null.

As a solution, a new function Var.getNonnull() could be added to Reference:

@Nonnull
public T getNonnull() {
    if (value == null) {
        throw new IllegalStateException();
    }
    return value;
}

[fge]

Good idea... Done.

[fge]

Actually, I'll leave this issue open until the next version is out.