feat(type-definitions): Add type guard filter type definition variant (#460)
feat(type-definitions): Add type guard filter type definition variant (#460)

If someone is using a typeguard, this changes the type of the stream and makes it easier to use. Example:

```js
filter(isString, from([1, 2, 'hello']))
  .map(v => {}) // v at this point is inferred to be a string rather than string | number
```
cujojs/most