User overridable panic builtin

[giann]

• Callback called when unrecoverable error is raised (e.g. index out of bound, out of memory)

fun panic(str error, [str]? stack = null) > void;

• New std method to set a user defined panic function

import "std";

|...

setPanic(
    fun (str error, [str]? stack = null)
        -> print("Unrecoverable error {error} at: {stack}"
    )
);