Add a new command to extract expression into a fresh let binding

[Tim-ats-d]

This PR adds a new command refactoring-extract-region to merlin protocol which allows extracting an expression into a let binding. See the interface comments for supported cases

Implementation

The implementation of the substitution is a bit hacky and relies on string manipulation, especially when the generated binding is a and. I couldn't deal with typedtree locations for the generated code, so I had to make a few manual location calculations that I tried to make as explicit as possible. Please tell me if there's a better way to achieve it.

It still lacks a bit of testing, I'll be adding more in the next few days.

Misc

I'have also added a fold function in Std.Option for the sake of convenience

 let fold ~none ~some = function
    | Some v -> some v
    | None -> none

cc @xvw @voodoos

[Tim-ats-d]

Just realized that I should probably add the corresponding documentation in the /doc/dev/PROTOCOL.md file