Allow lazy evaluation of some expressions

This is an internal requirement for features including:

- #201: User-defined functions (e.g. `def f(x) = x * n` where `f(5)` will multiply by whatever `n` currently is)
- #560: `WHILE` loops
- #619: Short-circuiting `&&` and `||` (so `2+2==4 || foo/bar==1` will evaluate to `1` even if `foo` is undefined or `bar` is 0)
- #621: Ternary operator (`cond ? then : else`)

Start by reading the linked issues #201, #560, #619, and #621 to understand the expression-evaluation requirements they describe. The issue does not name implementation files, tests, entry points, or a more specific completion criterion; done would require supporting the listed lazy-evaluation use cases.

Written by the indexing model from the issue text.