BTreeMap: Support custom allocators

[TennyZhuang]

I take over the task from an inactive PR #77438, and most of the work is done by the original author @exrook, thanks! If you are back, feel free to checkout the commits here and close my PR.

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Related:
rust-lang/wg-allocators#7
https://github.com/TimDiekmann/alloc-wg

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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[rust-highfive]

r? @m-ou-se

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[TennyZhuang]

r? @Amanieu

[Amanieu]

These changes to Cargo.lock seem to be unintentional, could you revert them?

[TennyZhuang]

It always be updated automatically when I do any x.py operations. 😄

[TennyZhuang]

@Amanieu can you help to reopen and review #77438 ? The original author is back and will take over it, I’d like to close the PR.