Support various kinds of rekeying/renaming keys

[rofinn]

I decided to go with pairs vs kwargs because:

1. I wanted to support unnamed dims
2. The nested pair syntax of oldname => newname => newvalues seemed more intuitive than mixing pairs and kwargs.

Closes #60

[rofinn]

Nightly error seems unrelated?

[glennmoy]

Nightly error seems unrelated?

nightly failed on my PR due to https://github.com/mcabbott/AxisKeys.jl/issues/57

[mcabbott]

Failure in 1.6 is

selectors: Error During Test at /home/runner/work/AxisKeys.jl/AxisKeys.jl/test/_basic.jl:106
128
  Test threw exception
129
  Expression: [V4[Near(x)] for x = xs] == [V5[Near(x)] for x = xs]
130
  BoundsError: attempt to access 10-element StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}} at index [0]
131
  Stacktrace:
132
    [1] throw_boundserror(A::StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}}, I::Tuple{Int64})
133
      @ Base ./abstractarray.jl:651
134

Seems like that should be unrelated... I should take a look at some point.

[mzgubic]

@mcabbott this would be nice to have, any chance you will have time to review at some point?

[rofinn]

Will likely fail CI with the new CanonicalIndexError on 1.8.