- building the necessary plumbing to support more functions with
  decent runtime performance. This is also necessary towards the
  long-term goal of loadable library modules.
- added new RainerScript function "tolower"

- implemented function support in RainerScript. That means the engine
  parses and compile functions, as well as executes a few build-in
  ones. Dynamic loading and registration of functions is not yet
  supported - but we now have a good foundation to do that later on.
  NOTE: nested function calls are not yet supported due to a design
  issue with the function call VM instruction set design.
- implemented the strlen() RainerScript function