Although it's supposedly part of `glib-2.0.vapi`, `string.replace` is
_not_ a function implemented by GLib. Worse, what it does is to create
and compile a `GRegex`, which then gets used to do the substitution.

Since we call `Persona.build_uid()` for each persona, and then even 3
times in a row, we create an amount of unnecessary temporary
allocations that linearly increases with the amount of contacts.

To mitigate this, use a `GString` (aka `GLib.StringBuilder`) and try to
allocate the right amount from the start.