Add methods (#2)

* stream add methods:
OfSlice, OfMap

* feat(stream): add methods

Stream interface: ReduceBy
factory methods: OfInts, OfInt64s,
OfFloat32s, OfFloat64s, OfStrings

* docs(readme): update readme

add changelog and todo;

stream add methods:

OfSlice, OfMap

Go stream. v0.0.1