component::Columns struct

[Anders429]

It's a common pattern throughout all of the Registry trait functions to pass component columns along with a length, and in each method those raw values are used to create a Vec<C> for each component C specified by the archetype::Identifier. This is fine, but we end up with a lot of unsafe code being repeated in every single method. For example, in src/registry/seal/storage.rs, Vec::<C>::from_raw_parts is called 8 times, which is in 2/3rds of the functions provided in that module.

A component::Columns abstraction might help here. Rather than having to deal with the raw parts directly and risk mishandling them, the pointers, capacities, and shared len could be placed in a single struct that could handle indexing into it, iterating over it, getting entity::Identifiers, etc. When iterating, a component::Column type could be returned which allows for unsafe casting to a Vec<C> by simply providing the type to a method.

Some investigation will need to be done to ensure this actually will reduce complexity of the code. It may be difficult to actually reduce the complexity, and we don't want to make things more complicated by introducing more modules needlessly.