Fix pop() corrupting index map when removing non-last element

[gaoflow]

Fixes #83.

After pop(index) removed an element from the middle of an OrderedSet,
the internal index map (self.map) held stale positions for every
element that shifted down. For example:

a = OrderedSet(["a", "b", "c"])
a.pop(0)     # removes "a"
a.items      # ["b", "c"]  -- correct
a.map        # {"b": 1, "c": 2}  -- still has old indices!
a.index("b") # returns 1 instead of 0
a[1]         # returns "c" instead of "b"

The discard() method already handled this correctly. This PR
brings pop() in line with discard() by rebuilding the map after
removal so that items[i] and map always agree.

Created under human direction with AI assistance.