pad this string at the beginning and at the end with `p` such that its `codepoint_count` is at least `n`.

In case the required number of copies of `p` is odd, the padding at the end will be longer.

pre
  safety: p.codepoint_count > 0
  debug: n >= 0

post
  debug: result.codepoint_count >= n
  debug: ((result.codepoint_count >= n+p.codepoint_count): result = String.this)