Support pre-allocated Fun construction

[dlfivefifty]

The idea is to support, for example:

f = Fun(Chebyshev(), Array{Float64}(1_000_000))
x = Fun()

f .= cos.(1_000_000.*x)  # no allocation

this is useful for time-stepping, for example.

To make this work, we need:

[ ] Non-allocating points #531
[ ] In-place transforms (plan_transform!)
[ ] A good syntax for doing this from an anonymous function. Something like

Fun!(f, x-> cos(1_000_000x))