Reduce overhead of char[] creation

[mdeinum]

There are more locations which could benefit from not using a
toCharArray on a String, but rather use the charAt method from
the String itself. This to prevent an additional copy of the
char[] being created.

[mdeinum]

Additional note, while inspecting the AspectJAdviceParameterNameDiscoverer I noticed that it might be beneficial to extract the isVariableName method from the AbstractAspectJAdvice to a helper/utility class and use that from both classes. They share the same logic to determine if something is a variable name.