Add iterator function builders; use in string.js

This commit adds a number of new `⸺IteratorFunction` functions in a
new `iterable.js` module, for building “iterator functions” from
various kinds of iterable object (as well as generator functions, which
are not themselves iterable). These functions are useful
metaprogramming tools for creating iterators akin to those produced by
the native Ecmascript builtins like `Array::values` or `Map::entries`
when starting from an already‐iterable base value but needing to map it
somehow.

The existing functions `codeUnits`, `codepoints`, and `scalarValues`
already fell into this category and have been refactored to make use of
the new technique.