Investigate options for function inlining

[ianmackenzie]

Although the Elm compiler now compiles the representation of a Quantity down to a plain Int or Float when --optimize is used, there is still a performance overhead of using Quantity values since things like addition have to go through a function call (Quantity.plus) instead of just being the + operator in the generated JavaScript. It would be useful to review different JS optimizers to see if there is one that can reliably inline these kinds of functions to eliminate the function call overhead. Candidates include:

• Google Closure Compiler
• UglifyJS
• Prepack (focus on performance instead of just bundle size!)