Build ArrayBuffers in Block.mapStmt (#1669)

* Build ArrayBuffers in Block.mapStmt

* Have empty Block serialize as "skip"

The FIRRTL parser requires at least one indented line in each module.
Sometimes tests emit and parse modules with no contents; this ensures
there's always at least a "skip" in empty modules.

Also fix tests that expected certain skips

* Use var List as stack in Block.mapStmt impl

This replaces Iterator concatenation. In Scala 2.11, RHS recursion on
Iterators is not stack safe. This seems to have been fixed in 2.12 by
Scala PR 5033.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>