Bump indexmap version to 2.0

[nilehmann]

No description provided.

[jonhoo]

Unfortunately this is a breaking change because IndexMap appears in the public type signatures of

tracing-timing/src/layer.rs

Lines 65 to 67 in 0b7a696

	pub fn with_histograms<F, R>(&self, f: F) -> R
	where
	F: FnOnce(&mut HashMap<S::Id, IndexMap<E::Id, SyncHistogram<u64>, Hasher>>) -> R,

and

tracing-timing/src/subscriber.rs

Lines 90 to 93 in 0b7a696

	pub fn with_histograms<F, R>(&self, f: F) -> R
	where
	F: FnOnce(&mut HashMap<S::Id, IndexMap<E::Id, SyncHistogram<u64>, Hasher>>) -> R,
	{

I'm not opposed to making a breaking change, but then I'd really like to improve upon the API experience (ref #17 for example) in the same swoop, as that will probably also require a breaking change.