## Objective
- Clean up remaining references to the trait `FromResources`, which was replaced in favor of `FromWorld` during the ECS rework.
## Solution
- Remove the derive macro for `FromResources`
- Change doc references of `FromResources` to `FromWorld`
(this is the first item in #2576)
Problem:
- When using the 'as_crate' attribute, if 'as_crate' was empty, the only
error you would get is 'integer underflow'.
Solution:
- Provide an explicit check for the 'as_crate' attribute's token stream
to ensure the formatting is correct.
Note:
- Also reworked 'get_meta' by not making it call 'Manifest::find' twice.
There are cases where we want an enum variant name. Right now the only way to do that with rust's std is to derive Debug, but this will also print out the variant's fields. This creates the unfortunate situation where we need to manually write out each variant's string name (ex: in #1963), which is both boilerplate-ey and error-prone. Crates such as `strum` exist for this reason, but it includes a lot of code and complexity that we don't need.
This adds a dead-simple `EnumVariantMeta` derive that exposes `enum_variant_index` and `enum_variant_name` functions. This allows us to make cases like #1963 much cleaner (see the second commit). We might also be able to reuse this logic for `bevy_reflect` enum derives.
