Initial attempt to optimize anonymous range iteration

When iterating over a range such as:
  for i in 1..10 do ...

we end up constructing a range and then getting it's low and high values out.
The range creation adds additional overhead that is not needed and it
obfuscates the loop from the backend compiler (it probably can't figure out how
the number of iterations even though the chapel compiler knowns it at compile
time.)

Ideally we want to generate
  for i in 1..10
as
  for(i=1; i<=10; i+=1)

This removes the construction cost, and should allow the backend compiler to
optimize/auto-vectorize better.

To accomplish this, this work replaces anonymous ranges with direct iterator
calls in for loops. Ranges are iterable because they have a these method. This
replaces the range with a direct call to an iterator. It essentially replaces
the code:
  for i in 1..10
with:
  for i in _opt_range_iter(1, 10)

the end result is that
  for i in 1..10
is now generated as:
  int64_t i;
  for (i = INT64(1); ((i <= INT64(10))); i += INT64(1)) {}
which is exactly what we wanted

This also handles anonymous ranges whose stride is known at compile time

E.G it handles
  for i in 1..10 by 2

and it will call into a more optimized iterator

if the stride is not known at compile time we don't do anything because we need
the range iterator to use chpl__diffMod

Not that this does nothing for aligned ranges, and doesn't handle cases such as
for i in 1..10 by 2 by 2 since it doesn't seem like a common case

The first attempt to do this was to replace the entire for loop with a c for
loop directly, but you can't do that for zippered iterators and figuring out
the type of the stride is really difficult and you have to insert calls to the
overflow checking and the general iterator.

A better long term solution might be to expand scalarReplacementOfAggregates to
includes ranges. That would allow all ranges to be optimized, including
non-anonymous ranges, and ranges that are used in contexts other than direct
iteration.

I still need to clean up the direct iterators, and make use of them from other
iterators. I also need to add bounds checks and the general iterator around it.