Loop type unification

[ScottCarda-MS]

We need a pass to convert the different types of loops in Q# into one consolidated type of loop or loop-representation in the HIR.

[ScottCarda-MS]

I think we might need to support RangeAsIntArray so that we can convert Range objects into something whose length is determined.

[ScottCarda-MS]

I think while loops are a good candidate for a unified loop.

[ScottCarda-MS]

Loops will be converted to while loops:

for <pat> in <expr> { <stmts> } will be handled differently depending on the type of <expr>.
If <expr> is an Array typed expression:

let __ary__ = <expr>;
let __len__ = __ary__::Length;
mutable __index__ = 0;
while __index__ < __len__ {
    let <pat> = __ary__[__index__];
    <stmts>
    set __index__ += 1;
}

If <expr> is a Range typed expression:

let __range__ = <expr>;
mutable __index__ = __range__::Start;
let __step__ = __range__::Step;
let __end__ = __range__::End;
while (__step__ > 0 and __index__ <= __end__) or (__step__ < 0 and __index__ >= __end__) {
    let <pat> = __index__;
    <stmts>
    set __index__ += __step__;
}

Other types for <expr> are invalid and will be caught in type-checking.

repeat { <repeat stmts> } until <cond> fixup { <fixup stmts> }
becomes

mutable __continue_cond__ = true;
while __continue_cond__ {	
    <repeat stmts>
    set __continue_cond__ = !<cond>;
    if __continue_cond__ {
        <fixup stmts>
    }
}