- add `[NotNull]` in some `public` or `protected` callers as well
- add `[NotNull]` in `SeekableTextReader` constructors
- add `where TSymbolType : struct` to replace incorrect `null` checks
- remove `T` type parameters in changed files e.g. change to `TWriter`
- remove tests of removed code
nits:
- change `TextReaderExtensions` to consistently call other extensions as statics
- wrap some long doc comments