Amp provides two common transformation helpers for iterators: `map` and `filter`.
Further primitives are very easy to implement using `Producer` with those two as examples.
## `map()`
`map()` accepts an `Iterator` and a `callable``$onEmit` that can transform each value into another value.
## `filter()`
`filter()` accepts an `Iterator` and a `callable``$filter`. If `$filter($value)` returns `false` the value gets filtered, otherwise the value is retained in the resulting `Iterator`.
