Adding support for overriding functions in Kotlin

[emarteca]

Adding support for override for generated kotlin functions that have the compatible signature of the function they should be overriding.
Right now just adding support for overriding toString.
This means that, if a Kotlin function with the signature fun toString(): String is generated, override is added to it so that it overrides the default toString.

This is specifically useful for allowing default printing of objects. If a Kotlin Result has an error type that overrides toString, then that toString is called to display the error, which is much more useful than the default toString that just prints the object address. With this PR, we can customize what's printed with the toString if the Result has a Rust-backed error type (by writing a valid to_string on the Rust side).

[Manishearth]

Not opposed to this way of solving this, but we already have the stringifier attribute, can we use that and have any method with stringifier automatically have its signature set to override fun toString()?

[emarteca]

Oh, good point! I didn't find the stringifier attribute, I'll use that instead.