Introduce checked functions for pushing, joining, and switching encodings

[chipsenkbeil]

Resolves #22 and #19.

Checklist

• push_checked
  • src/common/non_utf8.rs trait def
  • src/common/utf8.rs trait def
  • src/common/non_utf8/pathbuf.rs fn impl
  • src/common/utf8/pathbuf.rs fn impl
  • src/unix/non_utf8.rs trait impl
  • src/unix/utf8.rs trait impl
  • src/windows/non_utf8.rs trait impl
  • src/windows/utf8.rs trait impl
  • src/typed/non_utf8/pathbuf.rs fn impl
  • src/typed/utf8/pathbuf.rs fn impl
• join_checked
  • src/common/non_utf8/path.rs fn impl
  • src/common/utf8/path.rs fn impl
  • src/typed/non_utf8/path.rs fn impl
  • src/typed/utf8/path.rs fn impl
  • src/typed/non_utf8/pathbuf.rs fn impl
  • src/typed/utf8/pathbuf.rs fn impl
• with_encoding_checked
  • src/common/non_utf8/path.rs fn impl
  • src/common/utf8/path.rs fn impl
• with_unix_encoding_checked
  • src/common/unix/non_utf8.rs fn impl
  • src/common/unix/utf8.rs fn impl
  • src/typed/non_utf8/path.rs fn impl
  • src/typed/non_utf8/pathbuf.rs fn impl
  • src/typed/utf8/path.rs fn impl
  • src/typed/utf8/pathbuf.rs fn impl
• with_windows_encoding_checked
  • src/common/windows/non_utf8.rs fn impl
  • src/common/windows/utf8.rs fn impl
  • src/typed/non_utf8/path.rs fn impl
  • src/typed/non_utf8/pathbuf.rs fn impl
  • src/typed/utf8/path.rs fn impl
  • src/typed/utf8/pathbuf.rs fn impl