Richer Source Positions

[Pat-Lafon]

Now that there are a couple of front-end compilers, specifically looking at #227 and #197, I want to tie things together into a nice bow by extending #161 with (optional) richer source position information as discussed in part in #191.

To support something span-like, this pr adds an optional pos_end field to describe the end line/column of the position.

To provide source code for the error message, this pr adds an optional src field which is an absolute path to the original file that this position points to. Note that the src field does not need to be the same across the entire Bril program.

Each is currently described as their own field to be compatible with the original specification, but I would probably like to combine them into one object if that's fine. Something like

{
  ...
  "pos": {
    "pos_start" : {"row": <int>, "col": <int>},
    "pos_end" : {"row": <int>, "col": <int>}?,
    "src" : "<string>"?,
  }
}

[sampsyo]

Nice!! Pretty rad that this extra information can be done in a clean backwards-compatible way. Looks perfect to me!