Use double colon syntax to call static methods

[cburgdorf]

What is wrong?

When #239 lands we'll have support for SomeContract.create(..) and SomeContract.create2(..) which can both be thought as being static methods on a contract type. We should change static call syntax to use double colon syntax instead. To stick to the given examples, they would look like SomeContract::create(..) and SomeContract::create2(..)

Rational:

1. It clearly separates instance function calls from static function calls
2. It follows Rust syntax which we prefer to use over Python syntax in situations where we believe Rust syntax brings a clear benefit to the table

This will also work nicely with struct factory methods as demonstrated with the following example taken from #96

struct Rectangle:
    width: u256
    height: u256

# Static method to create a rectangle that happens to be a square    
def square(size: u256) -> Rectangle:
    Rectangle(width=size, height: size)

# An instance method to check if the rectangle can hold another rectangle
def can_hold(self, other: Rectangle) -> bool:
    return self.width > other.width and self.height > other.height

Taking self as the first parameter is what would decide whether something is a static method or an instance method (just like in Rust)

Usage:

big = Rectangle(width=10, height=20)
square = Rectangle::square(5)
big.can_hold(square)