Callable: restore name in disengaged mode, add caching

[Bromeon]

Pull request #1331 performed an optimization for Callables, but changed observable semantics in safeguards-disengaged configuration: the callable name is removed. This may need some more discussion, as there's a small change that some game logic relies on the name (e.g. when stringifying a Callable in GDScript).

This changes the approach, however performs different optimizations:

• Use CowStr, aka Cow<'static, str> instead of GString when constructing callables
• Only convert to GString (expensive) lazily, when actually used
• Cache the GString as well, to avoid recreating it

I measured the #[bench] performance compared to master in Release mode, with both balanced and disengaged safeguards. These are always minimum times.

Benchmark	Current BALANCED	Master BALANCED	Current DISENGAGED	Master DISENGAGED
callable_callv_custom	0.067μs	0.065μs	0.039μs	0.031μs 🥀
callable_callv_rust_fn	0.061μs 🌳	0.152μs	0.054μs	0.030μs 🥀
callable_to_string_custom	0.108μs	0.111μs	0.077μs 🌳	0.084μs
callable_to_string_rust_fn	0.084μs 🌳	0.096μs	0.055μs 🌳	0.070μs

Conclusion for this branch:

• 🌳 to_string is equally fast or faster across the board
• 🌳 calling is much faster in balanced mode for closures (no change for custom)
• 🥀 calling is significantly slower in disengaged mode

I would probably merge this due to quite a few other upcoming changes in Callable, but we should maybe revisit perf in disengaged mode. Many people do indeed not care about the callable name, and I don't like that being the thing slowing things down. But I'm also not sure if disengaged should behave differently, this could set a dangerous precedent.

It's quite possible that there are other optimization opportunities, or we make the name behavior more explicit.

cc @lyonbeckers