Generate intermediate AST for elm-deps

[wende]

Since parsing takes most of the time (Compiling Elchemy takes 29 minutes for parsing and 10 seconds for compiling). It'd be a good choice to cache files from elm-deps and save their AST instead of parsing them each time.

Stuff to be done:

1. Modify compiler to take saved ExContext.Context
2. Find a way to serialize and deserialize AST (https://github.com/mpdairy/elm-frontier ?)
3. Use serialized AST when file exists or parse the file otherwise
4. Add elchemy deps to recompile dependencies in case of changed dependencies

[wende]

This might help a lot
https://stackoverflow.com/a/14802843/2086804

[wende]

After some thought's, the only stuff that has to be exported are:

Modules, TypeDeclarations and TypeAliases.

Adding to the compiler an check abstractly represented by something like this would be sufficient:

1. Check if file elm/NAME.elm newer than lib/NAME.elchemy.ex
   a) if yes then compile the entire thing
   b) if no then take only module, type and type aliases and somehow inject them into the starting context. One way of doing that would be a special way of telling to the compiler it's a meta file and shouldn't have an output
2. Compile files to new ex files, but only if they have changed

Gains:

• MUCH better compilation time
• Compile deps once, use them forever