feat(core): support refinements in filter operator

[raveclassic]

Hi, this PR adds support for type refinements in filter operator:

type Foo = { foo: string }
const isFoo = (obj: any): obj is Foo => typeof obj['foo'] === 'string';
declare const source: Stream<unknown>;
const foo = filter(isFoo)(source) // Stream<Foo>

[briancavalier]

👍 Thanks!