Unsigned integers improvement and fixes

[Shaikh-Ubaid]

On main:

$ cat examples/expr2.py 
from lpython import u32, u64

def test():
    x : u32 = u32(-10)
    print(x)
    assert x == u32(4294967286)

    z: u64 = u64(x)
    print(z)
    assert z == u64(4294967286)

test()
$ python examples/expr2.py     
4294967286
4294967286
$ lpython examples/expr2.py
4294967286
18446744073709551606
AssertionError

On branch:

$ cat examples/expr2.py     
from lpython import u32, u64

def test():
    x : u32 = u32(-10)
    print(x)
    assert x == u32(4294967286)

    z: u64 = u64(x)
    print(z)
    assert z == u64(4294967286)

test()
$ python examples/expr2.py     
4294967286
4294967286
$ lpython examples/expr2.py
4294967286
4294967286

[certik]

Thanks!