Fast Evaluator should allow Functional Interfaces

[otbutz]

Example:

DoubleUnaryOperator op = new ExpressionEvaluator().createFastEvaluator("a * 0.9", DoubleUnaryOperator.class, "a");
double value = op.applyAsDouble(100);
System.out.println(value);

We should relax the current check to follow the JLS definition of Functional Interfaces instead:

A functional interface is an interface that has just one abstract method (aside from the methods of Object), and thus represents a single function contract.

        for (Method method : interfaceToImplement.getDeclaredMethods()) {
            int mod = method.getModifiers();
            if (method.isDefault() || Modifier.isStatic(mod) || !Modifier.isPublic(mod)) {
                continue;
            }
            if (methodToImplement != null) {
                throw new InternalCompilerException(
                        "Interface \""
                                + interfaceToImplement
                                + "\" must declare exactly one abstract method"
                );
            }
            methodToImplement = method;
        }