Proposal: Flatten `AllPallets` and similar types (#11813)

* flratten AllPallets types

* feature flag it

* fix

* fix

* fmt

* remove todo

* Update frame/support/src/traits/metadata.rs

Co-authored-by: Bastian Köcher <[email protected]>

* Update frame/support/src/migrations.rs

Co-authored-by: Bastian Köcher <[email protected]>

* fix

* mark as deprecated

* add docs

* fix ui test?

* fmt

Co-authored-by: parity-processbot <>
Co-authored-by: Bastian Köcher <[email protected]>