Reduce allocations in TypingTransformer based phases

[retronym]

Fixes scala/scala-dev#519

0.977693534x allocation reduction for compiling catsJVM

[info] HotScalacBenchmark.compile:·gc.alloc.rate.norm                       ../corpus           latest                    false  2.13.2-bin-05d028e-SNAPSHOT  @/Users/jz/code/cats/core/.jvm/target/cats-core-compile.args  sample   25  2558738109.440 ±   4386033.970    B/op
[info] HotScalacBenchmark.compile:·gc.alloc.rate.norm                       ../corpus           latest                    false  2.13.2-bin-4f284a8-SNAPSHOT  @/Users/jz/code/cats/core/.jvm/target/cats-core-compile.args  sample   25  2500535622.400 ±   2013538.465    B/op

TODO:

• Make LightTypingTransformer an abstract class rather than a trait to speed up the type test.
• Document the reason it isn't used for superaccessors (that stores local typers). Or maybe look at refactoring superaccessors to be less weird.
• Think about patmat's current reliance on implicit search working in typechecking for Array patterns. To witness this, modify typer to disable implicit search after the typer global phase. After this change, LightTypingTransformer based localTyper-s would no longer pick up in scope implicits contributed by enclosing members. It already ignores in-scope implicits that were made available via imports, as the Import nodes are dropped in refchecks.

[retronym]

@adriaanm @hrhino Just FYI, this is the work I mentioned yesterday.