Computed expressions respect left-to-right associativity and operator precedence

[sc1f]

This PR fixes #1072 by rewriting the expression parser to allow arbitary operator associativity without parentheses, as well as correct operator and function precedence: a + b * c ^ d / e, for example, evaluates to a + (b * ((c ^ d) / e)). This allows expressions to be much more complex and arbitrary than previously possible:

Changes

• Expressions respect left to right associativity and operator precedence
• Functions can have arbitary expressions inside them without additional parentheses
• Expression editor no longer destroys already-rendered tokens when an error occurs - so text no longer moves under the cursor while typing
• Adds parser + autocomplete tests
• Adds invariant tests for the parser
• Cleans and refactors the expression editor/parser

[texodus]

Looks good!

[texodus]

Thanks for the PR!