Implement slice object

[HyeockJinKim]

Currently the slice object is not fully implemented.
Many features, including instance creation, are not fully implemented and must be implemented.

gpython/builtin/builtin.go

Line 91 in 8c361a8

// "slice": py.SliceType,

gpython/py/slice.go

Lines 16 to 18 in 8c361a8

	var SliceType = NewType("slice", `slice(stop) -> slice object
	"slice(stop)
	slice(start, stop[, step])

Expected result (cpython)

>>> slice(10)
slice(None, 10, None)
>>> slice(10).indices(0)
(0, 0, 1)