Use hashbrown for std in addition to no_std builds

[ghost]

This simplifies dependency management a little bit and might also yield minimal performance improvements for large modules even though the included benchmarks do not have enough exports to make this measurable.

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[pepyakin]

The thing is, we most likely want to use BTreeMap even for no_std builds for some use-cases (see #183).

[ghost]

The thing is, we most likely want to use BTreeMap even for no_std builds for some use-cases (see #183).

I see. Is BTreeMap usable with no_std out of the box? If so, shall I just retarget this to replace HashMap by BTreeMap?

[pepyakin]

I see. Is BTreeMap usable with no_std out of the box?

Well, it is available with alloc create, but this is the case for hashbrown as well.

If so, shall I just retarget this to replace HashMap by BTreeMap?

I think it depends on your use-case: we want to use BTreeMap for now because of it's worst-case performance.

[ghost]

I think it depends on your use-case: we want to use BTreeMap for now because of it's worst-case performance.

Personally, I am interested in an embedded use case where I expect fewer than 10 exports per module and hence BTreeMap is fine for me (the compactness of an open-addressed hash table would be nice, but it really won't matter for me).

This MR was really just motivated by simplifying the build system a tiny bit. Hence I'll retarget this to use alloc::collections::btree_map::BTreeMap...

[ghost]

Closing in favor of #187