Implement WeakMap cache for frequent operations on data structures

[StarScape]

There are certain operations on Droplet's core data structures (paragraphs, runs, docs) that are extremely common and at least mildly expensive to compute. For example, text-len. We do not want to pre-calculate and store this value every time we instantiate e.g. a paragraph. It would be nice if instead we had a way of lazily calculating the value the first time it was needed, and then storing it the result, similar to this idiom in JS:

class Foo {
  get someValue() {
    if (this.__someValue !== undefined) {
      this.__someValue = someExpensiveComputation()
    }
    return this.__someValue
  }
}

Turns out you can solve this by use a cache based on a WeakMap of objects. Implement a weak-cache helper and cacheify any and all operations like this.