Adding support for stringifier functions in Kotlin

[emarteca]

Adding support for stringifier functions in Kotlin. This means, generating override fun toString(): String as the signature for a function annotated with the stringifier attribute.

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.