fix typings for later TS versions (3.4)

[wclr]

const mm$ = xs.createWithMemory<MemoryStream>()
const sm$ = xs.create<MemoryStream>()
const ms$ = xs.createWithMemory<Stream>()
const ss$ = xs.create<Stream>()
const fmm$ = mm$.flatten() // without fix this is typed to Stream<{}> not Stream
const fsm$ = sm$.flatten()
const fms$ = ms$.flatten()
const fss$ = ss$.flatten()

[staltz]

Thanks!

[staltz]

Released 11.11.0