Improve Internal Class-loading

This draft improves internal class-loading to make libraries more accessible from scripts and avoid using the Unsafe class injector unless necessary.

This draft also sanitises the AOT compilers to an extent, which should make errors a little less common.
Using the -Ddebug_mode=true flag at start-up will print full stack traces along with Skript errors.

Libraries now get their syntax loaded in preference to allow them to override default behaviour as they need to.