Migrating rust-analyzer assists to SyntaxEditor

This project aims to eliminate all usage of legacy mutable syntax APIs, incrementally update rowan and introduce necessary changes to RA, and then once all the new API's and changes to SyntaxEditor are fixed, we will move towards optimizing the implementation to make sure we are efficient Key objectives: 1. Migrate all remaining usages of mutable APIs (ted, make, clone_for_update). 2. Incremental rowan redesign to support: 1. trivia embedded in tokens 2. slot-based structure 3. SyntaxFactory generated from rowan grammar 4. Mutable API in accordance to named slots. 3. Evolve SyntaxEditor to operate new rowan. 4. Make rowan efficient.