Move #qualify_to and #qualify_to_first_source to extension

These are both basically aliases to #qualify, except instead of
an optional argument, they either require an argument or
require no arguments.  As they are rarely used, it doesn't make
sense to have separate methods for them.  The reason they exist
at all is they were added before #qualify.
Raise error for mismatched placeholders

A mismatched number of placeholders is indicative of some sort of
logic error.  However, especially for the case of two many
placeholders, it may not cause any problems, since additional
arguments are ignored.  So for a smooth transition, it needs to be
deprecated first.