Avoid closures for custom operators

To support custom operators for PowerPC and ARM, we will need to avoid using closures as these are not supported (https://github.com/JuliaLang/julia/pull/34326 and https://github.com/JuliaLang/julia/issues/32154). There might be performance advantages on other platforms as well.

We currently skip the relevant tests on these platforms (#353)

Some options:
- convert `Op(f, T=Any)` to a generated function
- write a macro that allows us to "declare" custom operator/type combinations, i.e.
```julia
@Op f T
```
would define a non-closure `cfunction` for `Op(::typeof(f),::Type{T})`