[core][flags] Add Flags type for storing information on memory layout

[forfudan]

Adds the Flags type for storing information on memory layout. It replaces the current Dict[String, Bool] type. The Flags object can also be accessed dictionary-like. Short names are available for convenience. It is similar to numpy.flags object. Example:

fn main() raises:
    var A = nm.random.rand(2, 3, 4)
    print(A.flags.C_CONTIGUOUS)
    print(A.flags["C_CONTIGUOUS"])
    print(A.flags["C"])

They all print True.