SI-8229 Source compatible name for implicit any2stringadd

To support the established pattern for disabling it for
an compilation unit.

Update scaladoc's knowledge of our "typeclasses".

Leave a `private[scala]` version of `StringAdd` (public in bytecode)
to ensure binary compatibility with 2.11.0-M8 for partest.
SI-7788 Avoid accidental shadowing of Predef.conforms

Rename `conforms` to `$conforms` and put in a minimal backstop: pos/t7788.scala

TODO: predicate the backwards compatibility shim for `Predef_conforms`
on `-Xsource:2.10`
Merge pull request #3555 from adriaanm/rebase-3553

Small Predef cleanup