BLI: generally improve C++ data structures

The main focus here was to improve the docs significantly. Furthermore,
I reimplemented `Set`, `Map` and `VectorSet`. They are now (usually)
faster, simpler and more customizable. I also rewrote `Stack` to make
it more efficient by avoiding unnecessary copies.

Thanks to everyone who helped with constructive feedback.

Approved by brecht and sybren.

Differential Revision: https://developer.blender.org/D7931

.clang-format

@@ -257,6 +257,10 @@ ForEachMacros:
   - SURFACE_QUAD_ITER_BEGIN
   - foreach
   - ED_screen_areas_iter
+  - SLOT_PROBING_BEGIN
+  - SET_SLOT_PROBING_BEGIN
+  - MAP_SLOT_PROBING_BEGIN
+  - VECTOR_SET_SLOT_PROBING_BEGIN
 
 # Use once we bump the minimum version to version 8.
 # # Without this string literals that in-line 'STRINGIFY' behave strangely (a bug?).