Fixing TooManyArgument and BadPassByReference warnings.

Refs #933.

- `_populate()` does not take an offset as an argument.
  The callee (`MultiKeyRecordSet::offsetGet()`) will work
  as intended nonetheless.

- Assign to a variable and cast before passing to `current()`.
Micro-optimize is_null usage.
Formatting.
Removing `extract()` usage where only minor changes are needed.

References #933.
Updating docblocks; adding missing options.