Missing type guard argument on fromFalsy

fromFalsy have this signature:

```typescript
fromFalsy<V>(val: V|null|undefined): Maybe<V>;
```

This means that the following produces `V=false|string`, where it should be `V=string`:
```typescript
fromFalsy(false | "some string")
```

I think the signature should be changed to:
```typescript
fromFalsy<V>(val: V|null|undefined|false): Maybe<V>;
```

