Reduce friction to use register_asset_type! macro

[ghost]

Feature

Stop requiring these lines of code to use the macro:

-> use amethyst::assets as amethyst_assets;
-> use amethyst_assets::register_asset_type;
-> use amethyst_assets::AssetProcessorSystem;
register_asset_type!(MyType => MyType; AssetProcessorSystem<MyType>);

Reason

High friction and cryptic error messages are bad.
Example error message:

error[E0433]: failed to resolve: use of undeclared crate or module `amethyst_assets`
  --> main.rs:64:1
   |
64 | register_asset_type!(MyUI => MyUI; AssetProcessorSystem<MyUI>);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `amethyst_assets`
   |
   = note: this error originates in the macro `register_asset_type` (in Nightly builds, run with -Z macro-backtrace for more info)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Maybe it's time to revisit this?