Integrating performance improvements from the Elm compiler into elm-format

Elm's de-facto standard source code formatter, elm-format is based on Elm's 0.15 compiler parsing code. Since Elm 0.19 the compilers parsing code has been rewritten to eliminate the dependency on parsec and indents for parsing and to greatly improve performance. elm-format has however diverged from the compiler to the point where integrating this rewrite is nontrivial. The end goal of this project is to integrate the performance improvements from the 0.19 compiler into elm-format.