Fix printing of unsigned integers

[certik]

def main():
    i: u8 = u8(255)
    print(i)

main()

This prints:

$ lpython a.py
-1

But should print 255.

[Smit-create]

So it needs a fix in the LLVM backend, and the fix basically requires keeping the value between [0, 2**(kind*8)). This should be handled at every assignment of unsigned integers and also while initializing. We need not worry about C backend as it will handle the same by itself.

This also needs some more fixes in LLVM because right now we treat i8 the same as u8 while i8 is unable to store 255. So we need to fix this by:

u_type = getIntType(max(a_kind*2, 8));

[certik]

LLVM treats u8 and i8 the same. It's the same LLVM type. The u8(255) is exactly the same bit representation as i8(-1). So I think the only fix we have to do is in the visit_Print() to treat the u8 correctly.

[Smit-create]

The fix I was suggesting not just only applies to print. If we fix just print there can be many other failures for the same reason. See the following:

def main():
    siz: Const[u8] = u8(128)
    ai16: i16[siz] = empty(siz, dtype=int16)

main()

This fails in LLVM but not in C because in LLVM siz is -1.

[certik]

The example you posted doesn't work: #1898. But assuming it "works", for now I would not allow unsigned integers in empty. So empty(siz) should give an error message in AST->ASR as well as in verify() that unsigned integer is not supported. In ASR this ends up being part of the "dimensions" and we should enforce in verify() that only signed integers are allowed. I made this #1899.