adapt_structure for StepRangeLen does not preserve eltype

[glwagner]

adapt_structure for StepRangeLen,

Adapt.jl/src/base.jl

Lines 79 to 80 in 5ef7c53

	adapt_structure(to, r::StepRangeLen) =
	StepRangeLen(adapt(to, r.ref), adapt(to, r.step), r.len, r.offset)

does not preserve the eltype of the range. For example:

julia> r = range(Float32(1), Float32(2), length=3); typeof(r)
StepRangeLen{Float32, Float64, Float64, Int64}

julia> using Adapt

julia> r_adapted = Adapt.adapt_structure(nothing, r); typeof(r_adapted)
StepRangeLen{Float64, Float64, Float64, Int64}

It's because the constructor called by adapt_structure has a different rule for working out the eltype than range, I suppose...

https://github.com/JuliaLang/julia/blob/9850a3881221a57a382e98c9b9ae2bf97ac3966d/base/range.jl#L524-L525

It's not a problem for Float64 ranges.

I think it's an easy fix by using the constructor StepRangeLen{T}(r.ref, r.step, r.len, r.offset).

[glwagner]

cc @ali-ramadhan

x-ref CliMA/Oceananigans.jl#3870