Use linear search for small IndexMap

[stepancheg]

When IndexMap is small, e. g. 5 elements, it is faster to:

• not allocate RawTable
• linear search elements

[cuviper]

I'm open to folks experimenting with this, but I think it will be nontrivial to plumb that condition through everywhere.

[cuviper]

I found halfbrown does this as an enum wrapper over hashbrown::HashMap.
Someone could do the same kind of thing with IndexMap.