Add structs using dataclasses

[certik]

We need to support structs using the following syntax:

@dataclass
class A:
    x: i32
    y: f32

def f(a: A):
    print(a.x)
    print(a.y)

def g():
    x: A = A()
    x.x = 5
    x.y = 5.5
    f(x)

g()

I think this already works at the ASR level and in LLVM, so the tasks are:

• Implement support for this at the AST->ASR level
• LLVM tests
• CPython tests
• C tests

[certik]

The basic features now work and are tested in CPython, LLVM and C.