using JS-style comments for {E,S}TcInsert

- HideInComment printing tree for dealing with tc-inserted things
- simple check to try single line comments (//: ...) if possible,
  otherwise multi-line (/*: ... */)
- since JS comments can't be nested, don't want to have nested TcInserts,
  which has been happening for hoisted scalar casts:
    [x = [cast](x)]
  so tweaking Typing.coerce to _not_ inserted ETcInsert, and it's up
  to the caller to do so if hoisting/backtracking is not being done.