Core: Remove implicit conversions from Callable and Signal to String, to avoid accidental conversions

[Ivorforce]

• Follow-up of Remove implicit conversions from math types to String, to avoid accidental conversions #107295

Same idea as above - we want to avoid accidentally converting to String, because it's expensive.

This also happens to fix another exact same type of bug as #107289, where Callable was accidentally converted to String before hashing, making using Callable as HashMap keys very slow. There is already a dedicated hash() function for Callable - it just wasn't used.

After this PR, there are just 4 implicit String conversions left:

• Variant - probably can't fix this now, Variant has all the implicit conversions.
• StringName - probably fine for now since it's usually fast (at least on repeated access).
• NodePath - there's a lot of accessors, so it needs its own PR.
• IPAddress - there's a lot of accessors, so it needs its own PR.

[Repiteo]

Thanks!