Support struct initialization with named arguments

[ubaidsk]

fixes #1800.

[ubaidsk]

Here, I was overlooking the first line if( x.n_keywords == 0 ) { \. Hence, I refactored this into a function, which I am hoping is more readable.

[ubaidsk]

I added some general error situations in tests/errors.

The working of named arguments is not yet perfect or exactly similar to cpython. The AST generated contains the m_args and m_keywords separately, even if the user source code contains them as mixed.

For example:

s = S(1, c=2, 3)

For the above, the ast contains args as [1, 3] and keywords as [(c, 2)]. This case in cpython would throw error that named argument is before positional argument, but the same example works in lpython. We are currently unable to catch such errors as the ordering information about args and keywords is not present in the AST, I think.

$ cat examples/expr2.py 
from lpython import i32, dataclass

@dataclass
class S:
    a: i32
    b: i32
    c: i32

def main0():
    s:S = S(1,c=2,3)
    print(s.a)
    print(s.b)
    print(s.c)

main0()
$ python examples/expr2.py                       
  File "/Users/ubaid/Desktop/OpenSource/lpython/examples/expr2.py", line 10
    s:S = S(1,c=2,3)
                   ^
SyntaxError: positional argument follows keyword argument
$ lpython examples/expr2.py                      
1
3
2

[ubaidsk]

Ready.

[certik]

Beautiful, thank you!