<functional> should avoid including <memory>

[StephanTLavavej]

Reported by: https://www.reddit.com/r/cpp/comments/wgb8xo/fun_times_with_msvc_functional/

Related to: #2996

<functional> was including <memory> in C++17 mode for unique_ptr and align() used by Boyer-Moore and _Ebco_base used by _Not_fn.

We can deal with align() and _Ebco_base by moving them up from <memory> to <xmemory>. This means that more code will drag them in, but they're quite small so this should have minimal impact.

For unique_ptr, we can replace it with a cut-down _Mini_ptr. I've added a comment mentioning that this has stripped away some of unique_ptr's misuse-resistance (e.g. it doesn't attempt to defend against Derived* being converted to Base* as a constructor parameter and then being array-deleted).

I've uglified _Get() and _Release(), and marked _Release() as _NODISCARD (which we've intentionally refrained from doing for unique_ptr since 10% of usage discards without leaking; here we control all usage).

For TUs (or Standard Library Modules) that include both <functional> and <memory>, the addition of _Mini_ptr is an added cost, but it should be quite small because the class is so cut-down. The savings for TUs that include <functional> without <memory> are substantial. Measuring #include <functional> with cl /EHsc /nologo /W4 /MTd /std:c++latest /P /showIncludes meow.cpp, I found that the preprocessed file shrinks:

• main: 2,157,990 bytes
• This PR: 1,882,819 bytes
  • Compared to main, this PR saves 275,171 bytes, which is 12.8% smaller. We avoid <memory> and <atomic> but still drag in <iosfwd> through the <xstring> path.
• <xhash> should avoid including <xstring> #2996: 1,945,700 bytes
• Both this PR and <xhash> should avoid including <xstring> #2996: 1,660,715 bytes
  • Compared to <xhash> should avoid including <xstring> #2996 alone, adding this PR saves 284,985 bytes, which is 14.6% smaller. The absolute improvement increases because we're able to fully avoid <iosfwd>.
  • Compared to main, both PRs together save 497,275 bytes, which is 23.0% smaller.

[StephanTLavavej]

I'm speculatively mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Resolved a merge conflict with #2806, same as in the mirror PR (keeping this PR's addition at the bottom).