Use objects instead of closures for type mappers (#36576)

* Use objects instead of closures for type mappers

* Flatten combined type mappers

* Single point of creation for type mappers

* More optimizations

* Fix lint error

* Fewer symbol instantiations / discard type mapper after instantiation

* More optimizations

* Simplify mapper layout and cache in composite mappers

* Removing cache as it doesn't seem to matter much

* Get rid of identityMapper

Author: ahejlsberg