Draft: AST visitor pattern

[fkleon]

Context

Implements the visitor pattern for the Expression AST.

Features and fixes

• fix: #90
• feat: Add support for listing variables in an Expression tree. #73
• feat: Add new real number evaluator.
• feat: Add new interval number evaluator.
• feat: Add new vector evaluator.
• feat: Require degree of Root function to be Expression (breaking).
• feat: Stronger typing for algorithmic function handlers.
• feat: cleanup evaluator tests; and improve test coverage for interval and vector evaluation types.