<functional> Implement bind_front()

[NathanSWard]

Description

This is an implementation of std::bind_front(). Addresses issue #13

Checklist

• I understand README.md. I also understand that acceptance of
  community PRs will be delayed until the test and CI systems are online.
• If this is a feature addition, that feature has been voted into the
  C++ Working Draft.
• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 .
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  NOTICE.txt), you must mention it here, so we can determine whether the
  license is compatible and what else needs to be done.

[msftclas]

All CLA requirements met.

[StephanTLavavej]

• You need to mention the new feature in yvals_core.h in the _HAS_CXX20 section in sorted order. We use the corresponding issue title, so it should be listed as P0356R5 bind_front().
• You need to update _MSVC_STL_UPDATE in yvals_core.h when adding a new feature, since it's a new month.
• You also need to define the feature-test macro __cpp_lib_bind_front in yvals_core.h.
• This should indeed be constexpr.

Excellent use of _NODISCARD - bind_front() should never be discarded (why call it?) but operator() can easily be discarded if the bound functor has useful side effects.

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

I believe that this will be ready to be checked in after these last remaining issues are fixed. Thanks!

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

I'm preparing a Microsoft-internal PR for this, #193, and #195, with associated test changes.

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

I believe that this is sufficiently clear now, although clang-format varies the wrapping a little (due to varying line lengths). At this point I believe there is no need for more wrapping as the repetitive structure is scannable enough.

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[CaseyCarter]

Approved assuming test coverage.

[StephanTLavavej]

While writing tests, I observe that WG21-P1651 claims "functors produced by std::bind_front are not assignable". I can't find any Standardese requiring them to be assignable, but I also can't find any Standardese requiring assignment to be ill-formed. Your bind_front() object behaves the same as the bind() object, so I think this is fine as-is.

[NathanSWard]

@StephanTLavavej Ok! Though, let me know if you'd rather have me explicitly put _Front_binder(const _Front_binder&) = delete;.

[StephanTLavavej]

Status update: I've sent out that Microsoft-internal PR with tests, so I should be able to merge this and the other PRs tomorrow.