Support for non-Float64 eltypes when using ForwardDiff

[torfjelde]

See tests for example + things like

x = ComponentVector(x = zeros(3))
∇ℓ = ADgradient(:ForwardDiff, ℓ; input=x)

also works.

[tpapp]

@devmotion: I am on holiday and will only check my emails occasionally in the next week or so, feel free to merge this and tag a release once your suggestions are implemented, so that TuringLang/AdvancedHMC.jl#301 is not held up.

[torfjelde]

Great! Made the changes:)