Variable handling in compiler is a mess

[phorward]

The current handling of locals, globals and temporaries is a mess. This became clear with a10b45a.
An entire re-design should be considered.

Some bad things to be resolved:

• Scope handling in compiler is ugly (started with Compiler scopes and intermediate object construction rework #132)
• Temporaries should be locals (on stack!)
• Globals should not be defined by compiler.local() on global scope
• ImlValue might not be the right choice for variables, maybe ImlVariable as internal information structure, to be used by e.g. compiler.temp() and compiler.untemp()?