Partially Resolves #197

Adds the --cue option to the eval utility. This allows jammed nouns to be passed over stdin and pretty-printed to stdout.

Also includes a sub-option which will expect the incoming jammed noun to be Khan output. If the thread resulted in a goof, it's pretty-printed to stderr as a stack trace and the output is simply the conn.c header info (unique ID, %avow mark signalling Khan output, %.n loobean signalling thread failure).

Also includes the "todo"s from the comments for #155.

Testing:

• Built Vere in debug mode; no new warnings introduced
• Manually ran the following test cases in the terminal:

echo '42' | ~/Urbit/urbit eval -jn | ~/Urbit/urbit eval -cn
echo '[[1 2] [3 4]]' | ~/Urbit/urbit eval -jn | ~/Urbit/urbit eval -cn
echo "(cat 3 'abc' 'def')" | ~/Urbit/urbit eval -jn | ~/Urbit/urbit eval -cn

• I've done more strenuous testing, but it's hard to explicitly list here since it relies on a thin client which will be committed as a separate PR after this one is merged

Not strictly necessary for this PR, but I would like to add explicit testing for both --jam and --cue. @mcevoypeter @matthew-levan If either of you can recommend how to modify Bazel to do so, it would be appreciated.