Add `rekey` method

[rofinn]

It's kind of annoying needing to reconstruct a KeyedArray every time you want to replace the keys in an array. I was thinking that we could do something similar to rename:

rekey(ka, (key1, key2, ...))
rekey(ka, dim1 => key1)

Maybe also:

rekey(ka, dim1 => (newdim, key1))

To do a rename and rekey in one operation?

NOTE: We're already doing a version of this in AxisSets.jl, but it'd be nice if we could just extend a method here.

[mcabbott]

Sounds good. Unlike rename, there's no confusion which way the arrow goes, so rekey(ka; dim1=kv1) would also be OK.