Configurable newline character (take 2)

[perlancar]

This is the continuation of issue #2.

I've sorted out the issues previously mentioned.

1. Comment '# tied Class' now not shown in single-line dumps.
2. Tests added for comment (in NL.t) and tied (in tied.t).
3. As for proper spacing/indenting in refcnt and require cases, we just need to set $NL to ' ' (single space) and $INDENT to '' (empty string). This is already documented.