@with_signature: Add `batch` option which enables multithreading of for loops using `Polyester.jl`

Usage:

@with_signature [batch=true] myfunction(eq::SomeEquation)
   @accepts ...
   # compute goes here
   @returns ...
end

I did some minimal profiling. There is a noticeable speed-up, but I did not measure how effective it is. For the time being, I am happy when it just runs a little faster.