Make Packed*Array API more consistent with Array

[Bromeon]

Somewhat follow-up to #720.

Adds Index/IndexMut for all packed arrays, allowing mutable/shared direct element access via p[index] operator.

Other changes (several breaking):

• get() now returns Option<T> instead of T
• deprecate set()
• rename binary_search() -> bsearch()
• optimize index access, since Godot already checks bounds and propagates that information
• methods that don't insert the element now take &T instead of T, consistent with Array
• changed method order, consistent with Array
• documentation

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-725