Implement for loops using scope injection

[c3d]

The current for loop definition is broken with recent evolutions of the language. This is the root cause for issue #4 and many other similar failures. The bottom line is that in the current implementation, the loop variable is unusable.

There was no way to properly inject a name in the body scope, so the loop was really "hacked" into the compiler in the historical Tao3D compiler, and did a number of things that are not as easy to do with the optimizing compiler, like having C++ code modify the values of variables directly.

The correct definition is now described in the documentation:

for N:name in R:[range of discrete] loop Body is 
    loop_context is 
        [[N]] : R.type := R.first 
    LoopVar is loop_context.[[N]] 
    while LoopVar <= R.last loop
        (loop_context) (Body) 
        ++LoopVar 

It uses a number of things that are not yet implemented and may need further clarification:

• The N:name notation to match a name in the parse tree should work even with the new type system, but was only tested in the old one.
• The [range of discrete] notation uses the range and discrete types, neither of which is presently implemented.
• The creation of a context like loop_context is only partially supported, and may need rework of the symbol table to fulfil its potential.
• The notation LoopVar is loop_context.[[N]] remains to be checked for feasibility. This usage of such an alias declaration would imply to be able to write into the alias. But so far, a lot of the documentation assumes that is can generally be implemented by generating a function. It's unclear if allowing an is definition to be used to write by default is a good idea. Maybe variable LoopVar is loop_context.[[N]] would be useful here.
• The (loop_context)(Body) notation injects a scope into Body, and is used to clarify that we are not looking for a loop_context prefix, but for a scope. Things should work without parenthese. Such use of declaration-only bodies as maps is documented but not implemented yet.
• The ++LoopVar notation uses a general increment operator for all discrete types that is not implemented yet.