Implement --infer #305

The --infer flag specified at the command line of lpython would infer the type of the variable from the RHS, thus allowing not to declare variable types in most cases. The return type of a function can be inferred from the "return" statements. The arguments of functions would still need to be typed, but later as we implement generics, say with syntax something like:

T = TypeVar("T")
def f(a: T, b: T):
    c = a + b
    return c

print(f(3, 4)) # would instantiate f as f(i32, i32) -> i32
print(f(3.5, 4.5)) # would instantiate f as f(f64, f64) -> f64

which with --infer would automatically infer the return type of "f" as T (as well as the type of c as T) and then the instantiation of the generics would correctly create full types at compile time.

Then we could go further, and --infer could simply make the following equivalent to the above:

def f(a, b):
    c = a + b
    return c

print(f(3, 4)) # would instantiate f as f(i32, i32) -> i32
print(f(3.5, 4.5)) # would instantiate f as f(f64, f64) -> f64

One tricky part is taht f(a, b) would technically be f(a: T, b: U), and I am currently not sure how one would infer the type of c = a + b. But maybe there is a way.

Initially --infer should not be on by default. It should be an opt-in feature, and we should gain experience with it and see if this is useful and if it can be on by default, or if will make Python code not as readable (such as like overuse of auto in C++). This option might be helpful for interactive use.