Use 'Items' and 'Collections' in uniques pallet (#11389)

* Rename class to collection

* Use "assets collection" instead of "asset collection"

* Rename 'instance' to 'asset'

* Change "asset `collection`" to "`collection`"

* A bit more clean up

* Rename Asset to Item

* Add a storage hack

* Typos

* fix compile

* fmt

* Fix

* cargo run --quiet --profile=production  --features=runtime-benchmarks --manifest-path=bin/node/cli/Cargo.toml -- benchmark pallet --chain=dev --steps=50 --repeat=20 --pallet=pallet_uniques --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --output=./frame/uniques/src/weights.rs --template=./.maintain/frame-weight-template.hbs

* Update frame/uniques/src/lib.rs

* cargo run --quiet --profile=production  --features=runtime-benchmarks --manifest-path=bin/node/cli/Cargo.toml -- benchmark pallet --chain=dev --steps=50 --repeat=20 --pallet=pallet_uniques --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --output=./frame/uniques/src/weights.rs --template=./.maintain/frame-weight-template.hbs

* Change 'items collection' to 'collection'

* Apply suggestions

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Parity Bot <admin@parity.io>