Rename Base.Order.Forward to lowercase

[jakobnissen]

Since Forward and Reverse are singleton instances like nothing and missing, it should probbaly have been named forwardordering and reverseordering instead for consistency with the Julia style guide and convention.

This is clearly breaking, but might be worth considering for 2.0.

[kimikage]

If we follow the guideline, the instances of RoundingMode (e.g. RoundNearest) should also be lowercase.
I do not strongly oppose changing them to lowercase. However, the changes reveal that they are not types, but make it difficult to distinguish them from ordinary variables or functions. I’m not sure that is quite right. 😕

[tkf]

Same for MergeSort, QuickSort, and InsertionSort.

[goretkin]

I'm concerned with the usability (especially for people who are not expert English readers) of introducing more names without delimited word boundaries in place of names with word boundaries.