Fix multiline docstrings

(I screwed up multiline docstrings. Sorry! I don't usually write ns docstrings, and the tests didn't cover this case. This patch should correct all of this.)

Commit e66a40683e2ebf94a1e3bea53706c55ab879c7cd fixed unintentional
unescaping of docstrings, but also broke multiline docstrings:

        (ns example
          "Foo
           Bar
           Baz")

        =>

        (ns example
          "Foo\n   Bar\n   Baz")

This approach manually escapes \ and " inside of docstrings, but leaves
newlines and other characters as is so that the string is reprinted as
intended.