Jonas Schievink 56e0806a1a
Rollup merge of #78417 - ssomers:btree_chop_up_2, r=Mark-Simulacrum
BTreeMap: split off most code of append

To complete #78056, move the last single-purpose pieces of code out of map.rs into a separate module. Also, tweaked documentation and safeness - I doubt think this code would be safe if the iterators passed in wouldn't be as sorted as the method says they should be - and bounds on MergeIterInner.

r? ```@Mark-Simulacrum```
2020-11-11 20:59:00 +01:00
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