Use "C for loop" in DefaultRectangular and remove posStrideIter

Default rectangular used to use a while loop in an iterator instead of
forwarding to ChapelRange as an optimization to avoid using a general strided
range iterator. With C for loops V2 this was changed to call into an optimized
range iterator for positively strided ranges. However, we noticed that for tests
like n-body we had a performance hit because of this.

The reason for this is because forwarding to ChapelRange introduces the
overhead of range construction. The posStridedIter also used alignedLow/High
which meant calls to chpl_diffMod.

In order to avoid these additional (and in the case of defaultrectangular
unnecessary) calls we are using a C for loop in DefaultRectangular. This is a
little unfortunate as we originally only wanted C for loops in ChapelRange, but
we don't envision needing them anywhere else. Since posStridedIter was only
around for DefaultRectangular, I'm removing that as well.
Update explain instantiation since it no longer instantiates another range
Update comment in DefaultRectangular with brads recommendation
Merge pull request #407 from ronawho/CForLoops-DefaultRectangular-Perf

Use "C for loop" in DefaultRectangular and remove posStrideIter

[reviewed by @bradcray]

Default rectangular used to use a while loop in an iterator instead of
forwarding to ChapelRange as an optimization to avoid using a general strided
range iterator. With C for loops V2 this was changed to call into an optimized
range iterator for positively strided ranges. However, we noticed that for tests
like n-body we had a performance hit because of this.

The reason for this is because forwarding to ChapelRange introduces the
overhead of range construction. The posStridedIter also used alignedLow/High
which meant calls to chpl_diffMod.

In order to avoid these additional (and in the case of defaultrectangular
unnecessary) calls we are using a C for loop in DefaultRectangular. This is a
little unfortunate as we originally only wanted C for loops in ChapelRange, but
we don't envision needing them anywhere else. Since posStridedIter was only
around for DefaultRectangular, I'm removing that as well.