[phobos] Transversal/Transposed

[David Simcha]

Last year, when I was cleaning up/debugging std.range, one issue I never 
got around to addressing was Transposed.  Currently it's completely 
undocumented even though it could be highly useful.  To get it up to 
snuff, I think I need to convert FrontTransversal into EndTransversal.  
EndTransversal would decide at runtime via an enum whether to iterate 
the front or back of the range of ranges:

auto ror = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
auto frontTrans = endTransversal(ror, TransversalType.front);
assert(equal(frontTrans, [1, 4, 7]));

auto backTrans = endTransversal(ror, TransversalType.back);
assert(equal(backTrans, [3, 6, 9]));

This would come at a small performance cost, since every call to 
EndTransversal.front would need to decide whether to fetch the front or 
back of each range.  It would not be possible to decide this at compile 
time because Transposed.back and Transposed.front need to return the 
same type.

Since this is going to require significant effort, is everyone on board 
with it before I get busy?