Fixes multiple issues with stack traces leading to missing
stack trace lines. Also, we no longer put builtin context
on the line which *calls* the builtin as if it was a part
of the builtin itself.
Code for stack trace handling was centralized. We no longer
need traceElement argument in ~every function. Now the stack
trace state is kept solely in the interpreter.
* Location, error formatting and stack trace improvements
* Static context for AST nodes
* Thunks no longer need `name`
* Prototype for showing snippets in error messages (old format still
available)
* Use ast.Function to represent methods and local function sugar.
* Change tests so that the error output is pretty
