Better CESK machine pretty-printing

[byorgey]

Closes #644. Currently this only helps for debugging, but a first step towards #5.

Some kind-of-randomly-selected before/after examples:

BEFORE:

▶ say
  [_ "Ready!" | exec _]
◀ say
  [_ "Ready!" | exec _]
▶ "Ready!"
  [say _ | exec _]
◀ "Ready!"
  [say _ | exec _]
◀ say "Ready!"
  [exec _]
◀ ()
  []

AFTER:

E· (▶say◀ "Ready!")
E· (◀say▶ "Ready!")
E· (say ▶"Ready!"◀)
E· (say ◀"Ready!"▶)
E· ◀say "Ready!"▶
◀()▶

BEFORE:

▶ c
  [exec _ | b <- _ ; if b {noop} {force until c} | _ ∪ <Env> | _ ; grab | _ ∪ <Env> | loadEnv]
◀ as <a1> {has "Win"}
  [exec _ | b <- _ ; if b {noop} {force until c} | _ ∪ <Env> | _ ; grab | _ ∪ <Env> | loadEnv]
◀ false
  [b <- _ ; if b {noop} {force until c} | _ ∪ <Env> | _ ; grab | _ ∪ <Env> | loadEnv]
▶ if b {noop} {force until c}
  [exec _ | _ ∪ <Env> | _ ; grab | _ ∪ <Env> | loadEnv]
▶ if b {noop}
  [_ {force until c} | exec _ | _ ∪ <Env> | _ ; grab | _ ∪ <Env> | loadEnv]
▶ if b
  [_ {noop} | _ {force until c} | exec _ | _ ∪ <Env> | _ ; grab | _ ∪ <Env> | loadEnv]

AFTER:

L· ∪· ((b <- E· ▶c◀ ; if b {noop} {force until c}) ; grab)
L· ∪· ((b <- E· ◀as <a1> {has "Win"}▶ ; if b {noop} {force until c}) ; grab)
L· ∪· ((b <- ◀false▶ ; if b {noop} {force until c}) ; grab)
L· ∪· (∪· E· ▶if b {noop} {force until c}◀ ; grab)
L· ∪· (∪· E· (▶if b {noop}◀ {force until c}) ; grab)
L· ∪· (∪· E· (▶if b◀ {noop} {force until c}) ; grab)

[xsebek]

This looks amazing! I really wish we could open some debugging window in the game and see all the robot frame updates during the last tick. 🤩