Figure Out How to Display Nodes in Diagnostics

[harlanhaskins]

Description

Right now, diagnostics contain a reference to a node in the Syntax tree or token stream. This is really powerful, because it means we can just walk up the tree to get more context when we want to print
an expression out -- and because of the libSyntax-style AST, we can faithfully reprint the code from the original file, with all whitespace intact, back to a string.

We need to find a good predictable set of rules for

• How far to walk up the tree for context
• What textual ornamentation we want for the code (^ and ~~~~~ for pointing and highlights?)
• How to highlight specific expressions in the printed code
• Any other special things we want to do (Rust has some really cool ASCII art diagnostics!)