reflect: `insert` and `remove` methods for `List`

[soqb]

What problem does this solve or what need does it fill?

Currently, the reflect List trait implements only push and pop functionality. There is no way to insert or remove elements other than from/to the end other than by downcasting. This is especially notable using types like VecDeque with reflection.

What solution would you like?

Add insert and remove methods for List and implement them.
Give push and pop default implementations using on the insert/remove and len methods.

What alternative(s) have you considered?

We might not need to implement the methods by default but since we can I believe we should as to not put to much burden on the implementor.

Additional context

Discussed first on discord.

[alice-i-cecile]

Yep, if we can have a default impl for this I'm on board. Probably warn in the trait docs that manual impls may be faster.

[soqb]

trying to implement this. for the signature of insert and remove, should it match Vec where insertion and removal panic when the index is out of bounds or have them as fallible operations: fn insert(...) -> bool and fn remove(...) -> Option<Box<dyn Reflect>>?

edit: push and pop both match Vec but push never panics or fails and pop returns an Option.

[MrGVSV]

trying to implement this. for the signature of insert and remove, should it match Vec where insertion and removal panic when the index is out of bounds or have them as fallible operations: fn insert(...) -> bool and fn remove(...) -> Option<Box>?

I’m in favor of just matching Vec. We have ways for users to reflectively perform their own bounds checking to ensure they don't panic, so I think it's fine to match that behavior