Auto merge of rust-lang#30426 - gereeter:btree-rewrite, r=Gankro

Despite being over 700 lines shorter, this implementation should use less memory than the previous one and is faster on at least insertions and iteration, the latter improving approximately 5x.

Technically a [breaking-change] due to removal of deprecated functions.

cc @apasel422 @gankro @goyox86

Fixes rust-lang#27865.

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Author: bors