Support generating properties for getters and setters

[randomPoison]

Right now all methods in an impl block are generated as methods in the corresponding C# class. For simple getters, though, the more idiomatic equivalent in C# would be to use a property. For example, in the following case:

impl PersonInfo {
    pub fn age(&self) -> u32 {
        self.age
    }
}

The idiomatic C# equivalent would be:

public int Age => ...;

Similarly, simple setters should also be generated as properties instead of methods.

In simple cases, it's not too difficult to automatically detect getters and setters: A method that takes no parameters and returns a single value is a getter, and a method that takes a single parameter and returns nothing is a setter. However not all methods that follow this pattern should be converted into properties: Generally methods that "look like" getters but perform expensive calculations are kept as methods in C# in order to encourage users to only call the method once and cache the result of the calculation. As such, we'll need to decide if we want to use this heuristic to optimistically generate properties on a best-effort basis, or if we should require users to annotate functions that should be made into properties.

Either way, we'll need attributes that allow users to manually specify when methods should (or should not) be generated as properties. I'm thinking:

• #[cs_bindgen(getter)] forces the method to be generated as a getter.
• #[cs_bindgen(setter)] forces the method to be generated as a setter.
• #[cs_bindgen(getter, name = "Foo")] and #[cs_bindgen(setter, name = "Foo")] explicitly set the name of the generated property. This allows users to make sure a getter/setter pair get matched together in a single property. Without this, cs-bindgen will attempt to automatically match getters and setters by naming convention, i.e. foo and set_foo will be matched up into a single property Foo.

The #[cs_bindgen(name = "Foo")] should probably be generalized such that it can be used to control the name of any generated item.