Add `#[inline]` to `BTreeMap::new` constructor
This PR add the `#[inline]` attribute to `BTreeMap::new` constructor as to make it eligible for inlining.
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For some context: I was profiling `rustc --check-cfg` with callgrind and due to the way we currently setup all the targets and we end-up calling `BTreeMap::new` multiple times for (nearly) all the targets. Adding the `#[inline]` attribute reduced the number of instructions needed.
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