Tweak mandelbrot-fast.jl for performance.

[non-Jedi]

Multiple versions with different threadings included because different versions are faster depending on the machine. Depending on machine, gains can be over 20% compared to original mandelbrot-fast.jl.

I would appreciate input on relative timings for each of the scripts others see on their computer. For example, I get the following timings on two of my machines using hyperfine:

	laptop with Intel(R) Core(TM) i5-5300U CPU @ 2.30GHz	desktop with Intel(R) Core(TM) i5-3470 CPU @ 3.20GHz	desktop with Intel(R) Core(TM) i5-6500 CPU @ 3.20GHz
mandelbrot-fast.jl (original)	2.492	1.864	1.788
mandelbrot-fast.jl	2.426	1.494	1.401
mandelbrot-fast.v2.jl	2.391	1.802	1.718
mandelbrot-fast.v3.jl	2.377	1.543	1.448
Best	mandelbrot-fast.v3.jl	mandelbrot-fast.jl	mandelbrot-fast.jl

NOTE: running mandelbrot-fast.v3.jl requires installation of https://github.com/mohamed82008/KissThreading.jl

Changes included in every version:

• Removing threading from filling xvals and yvals--threading overhead is too high for such a simple operation.
• Remove @simd annotation from mandel_inner--simd is occurring at the level of mand8; @simd doesn't hurt runtime but increases compilation time.
• Only run mandelbrot when !isinteractive() to make development and debugging easier.
• Various tweaks and minor stylistic updates for succinctness and maybe a marginal increase in performance.

[non-Jedi]

xref https://salsa.debian.org/benchmarksgame-team/benchmarksgame/issues/245