Add dict.keys

[kabra1110]

Issue: Some extra zeros in list.

from lpython import i32

def test_dict_keys_values():
    d: dict[i32, i32] = {1001:2002, 3003:4004}
    x: list[i32]
    x = d.keys()
    print(x)

test_dict_keys_values()

[0, 1001, 0, 3003, 0]

[certik]

I think this looks good. @kabra1110 is this ready?

[certik]

Ah I see, there are some extra zeros.

Make sure to test the result and make it work in both LPython and CPython.

[kabra1110]

Corrected this for linear probing implementation, and added dict.values for the same.

The test in f28eaf0 works with lpython, but not with python, as these functions do not return lists in the latter. I modified them to make copies into lists, but looks like there are some issues with nested iterables.