P1209R0 erase_if(), erase()

[SuperWig]

Description

Resolves #55.

Checklist

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[StephanTLavavej]

Thanks for working on this! If you can update this with the code movement and other relatively minor changes requested, we can write some tests and merge this.

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

Azure Pipelines is failing due to clang-format validation. Please run clang-format 9.0.0 before pushing the next round of updates.

For the <experimental/meow> changes, the addition of the deprecation macro is making those lines exceed our 120 column limit, causing clang-format to want to wrap them. For those functions, you can simply delete the comments saying // erase each element satisfying _Pred and so forth; they don't really provide any value (and our conventions regarding function comments have changed, so we no longer require them for simple functions).

[SuperWig]

I just added them to the end of because I didn't know where else to put it.
I also re-ordered the _Erase helpers, wasn't sure if that was wanted or not but it looked more consistent this way.

[CaseyCarter]

LGTM modulo test coverage.

[BillyONeal]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

This looks perfect, with one stylistic ordering nitpick. To save time, I'll push a change for that, then work on porting this over to our MSVC-internal repo and writing simple tests (plus changes needed to remove <experimental/xutility> from the VS installer). We should be able to merge this tomorrow if all goes well.

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

This passed all of our tests, including libcxx's tests (modulo some truncation warnings in their test code that we'll need to fix in the near future). Thanks again, and congratulations for implementing your first C++20 feature here!