unicycle: improve usability

[noelleleigh]

This PR makes several changes to Varvara's Unicycle to improve usability and accessibility:

• Added title attribute to the <iframe> and <title> to the document itself for accessibility.
• In emu.js, change Console.display to be a function instead of an HTMLElement to make it more flexible.
• unicycle/index.html:
  • Use <form> with a submit button to give users more than one way to execute the uxntal statement.
  • Use an <output> element to show the result of the uxntal evaluation in a more semantic way.
  • Style the input and output in monospace.
  • Change placeholder to be more instructive.

[neauoire]

Who's the best? Noelle is the best <3