Modules cdt and cdt2 do not work with latest devel compiler

[StefanSalewski]

There are two issues: A simple one:

Line 112 in minmax.nim

func minValueBy*[T](s: openArray[T], cmp: proc (x, y: T): int {.closure, noSideEffect.}): T =
#func minValueBy*[T](s: openArray[T], cmp: proc (x, y: T): int {.closure.}): T =

More recent compiler versions needs the noSideEffect pragma (or we may make minValueBy() a plain proc.) Making
the symbol after cmp: a func seems not to work.

The other issue is

grep cursor /home/salewski/.nimble/pkgs/cdt-0.1.1/cdt/*
/home/salewski/.nimble/pkgs/cdt-0.1.1/cdt/types.nim:    next*{.cursor.}: Edge # added {.cursor.} for v 0.1.1

We added the cursor pragma years ago to make it compile with --gc:arc. Orc was not available at all that time.
Now, we can remove the cursor pragma, and it compiles with orc, arc, and refc. So the obvious fix is to remove that pragma now. But with arc the cdt module may leak memory, due to the complicated quad-edge data structure. May the cursor pragma avoid leaks for arc? We don't know currently.

But to avoid the ugly crashes, we will remove the cursor pragma for now.