Improve the user-facing Carbon Explorer Output

Currently, Carbon Explorer's trace output is both verbose and hard to use. There are a large range of improvements that could be made here. This is especially true when considering its use as part of Compiler Explorer. Possible tasks include: Add a mode to only output the abstract machine execution trace and enable this by default. Add a mode to filter out the evaluation of the prelude and other imports and enable that by default. Include nicely rendered snippets of source code during the evaluation trace to highlight the code being evaluated at each step. Come up with a set of design improvements to the textual rendering to make it more comprehensible for people just arriving on the project and more easily scanned quickly. (Might involve fancier terminal output, colorized output, line drawing, etc.) Stretch goal: interactive tools to incrementally step as a precursor to the debugging features.