Using `similar` for preallocation

In [line 28](https://github.com/JuliaMath/AbstractFFTs.jl/blob/d007201e640b567a1a9888c478f46dd70004f511/src/definitions.jl#L28), whoever originally wrote this expressly went out of their way to avoid using `y = similar(x)`. I can't seem to find any discussion as to why this was done except for the vague comment, but I'm currently trying to interface with AbstractFFTs and this would be a huge minus for me.
Currently, I'm respecting whatever type the user inputs. While just plugging in AbstractFFTs would likely work just fine, I think this behavior could be undesirable. For example, right now, using `StaticVector` as both input and output accelerates the algorithms tremendously, which is particularly consequential when performing many very small FFTs. I asked @ararslan about this a few weeks ago, and got the response that this was ported over from when AbstractFFTs was in Base. Is it still necessary to do this?