Optimize SimpleFunctionMap

[sdwfrost]

Is your feature request related to a problem? Please describe.

Running a small function map example using SimpleFunctionMap is slower than a hand-coded loop by a reasonable margin.

Full examples:

• https://github.com/epirecipes/sir-julia/blob/master/markdown/function_map/function_map.md
• https://github.com/epirecipes/sir-julia/blob/master/markdown/function_map_vanilla/function_map_vanilla.md

Describe the solution you’d like

Options - perhaps at the cost of fewer safeguards - to improve the speed of the SimpleFunctionMap.

Describe alternatives you’ve considered

I haven't done an in-depth profile, but it seems like pre-allocating output for this example (as in my code) may help.