vNext Planning

Salvage The Old vNext Branch

Multithreading overhaul
Iterator debugging overhaul (doubly-linked iterator chains, no proxy allocations)
- Still need to overhaul deque and vector<bool>
- Also simplifies checked/unchecked and modifiable/const, avoiding inheritance (which is non-standard)
Remove /clr:pure
Flat C exports
Remove <hash_map>, <hash_set>
Remove tr1
Other changes?

vNext Issues
- #938 P0408R7 Efficient Access To basic_stringbuf's Buffer can be significantly simplified
TRANSITION, ABI comments
Consolidate satellite DLL and import LIB code back into the main DLL (and API loading code like for ICU); make sure to keep vector/string etc. out of the DLL/import LIB.
Eradicate the last traces of XP/Vista support
Remove _app DLLs
<variant> changes:
- Store index() + 1 instead of index() to so the range of possible stored values is contiguous for minor codegen improvements.
- The hash value of a variant should depend on its index() as well as the stored value so variant<int, int>{in_place_index<0>, 42} and variant<int, int>{in_place_index<1>, 42} don't collide.
[[no_unique_address]] will be unconditionally available, allowing us to replace _Compressed_pair and improve the representations of more types.
iomanip.cpp doesn't need to be separately compiled
Unnecessary padding in _Ctypevec (was #426)
Likely unnecessary padding in basic_istream
Ensure that future<const T> and promise<const T> work with the overhauled representation (was #2599)
We should regularly add exports to prevent people from assuming they can use old DLLs