Resolves #25 and resolves #26. Currently active for Clang and EDG, but not C1XX.

* Simplify thread0.cpp and xthrow.cpp.

I verified with `#error` that these files are always compiled with
`_HAS_EXCEPTIONS=1`.

thread0.cpp
Include only necessary headers.

Move the lookup tables within `_STD_BEGIN` to avoid repeated `_STD`
qualification. They're `static`, so this doesn't affect bincompat.

Add trailing commas for readability.

Make `codes` a table of `errc` so we don't need to cast each value.

Mark `_Throw_Cpp_error` as `[[noreturn]]`, matching its declaration
in the header. (No bincompat effect.)

Use `_THROW` for consistency. (Even though exceptions are enabled,
we conventionally use this in `src`.)

`system_error` is a type, so we don't need to `_STD` qualify it.

`static_cast<int>` the `errc`, once.

xthrow.cpp
Coalesce headers. Coalesce `std` regions.

Remove unnecessary comments.

Mark value parameters as const.

* Add constexpr.

…#244)

Xiang Fan of the C1XX frontend team reported that this improved throughput in a customer submitted benchmark.

Implements iterator primitive operations `std::ranges::advance`, `std::ranges::next`, `std::ranges::prev`, and `std::ranges::distance`; as well as `std::default_sentinel` and `std::unreachable_sentinel`.

This change reworks the STL's iterator unwrapping machinery to enable unwrapping of C++20 move-only single-pass iterators (and `if constepxr`s all the things). Consequently, `_Iter_ref_t`, `_Iter_value_t`, and `_Iter_diff_t` resolve to `iter_reference_t`, `iter_value_t`, and `iter_difference_t` (respectively) in `__cpp_lib_concepts` (soon to be C++20) mode. This change necessitates some fixes to `unique_copy` and `_Fill_memset_is_safe` which both assume that `_Iter_value_t<T>` is well-formed for any iterator `T`. (`iter_value_t<T>` does not have that property: it is only well-formed when `readable<T>`.)

I notably haven't unified `default_sentinel_t` with `_Default_sentinel` out of an abundance of paranoia. Our `move_iterator` is comparable with `_Default_sentinel`, which is not the case for `std::default_sentinel`.

Drive-by:
* This change `if constexpr`-izes `unique_copy`.

…ers (#341)

* Implement P1690R1 Refining Heterogeneous Lookup For Unordered Containers

* Mark P1690R1 as implemented in yvals_core.h.

Addresses #268 by changing the implementation to match the current Standardese.

Programs that include such specializations are ill-formed, since the Standard only specifies a partial specialization of `std::function<T>` for non-`noexcept` function types `T`. The current diagnostic:
```
error C2027: use of undefined type 'std::_Get_function_impl<_Fty>'
```
is not great.

Fixes DevCom-825902.

[This is a replay of Microsoft-internal MSVC-PR-215822.]

Hidden friends aren't hidden in C1XX's permissive mode, so let's use an alternate mechanism to make these operators truly ADL-only. (We want to avoid checking `weakly_incrementable` for every type that is compared via `==` or `!=` with a type associated with namespace `std`.)