Disallow mixing of sequential and named sprintf parameters

[HertzDevil]

Fixes the following for sprintf, IO#printf, and String#%:

• Format strings cannot contain both sequential parameters (e.g. %s) and named parameters (e.g. %{foo}, %<bar>s). They are detected as soon as they consume an argument.
• Calls that use named parameters in the format string must pass exactly a single Hash or NamedTuple, or a one-element Array or Tuple of it, depending on the overload chosen.
• As a result of the refactoring, % is no longer one of the allowed field types; exactly %% must be used to print the percent sign, and other format modifiers are not allowed. This is the behavior in Ruby but not C. This is justified because format strings like %<a>% do not make any sense.

The PR also lays the groundwork for #12305.