This PR addresses #8902 in two ways:

• Always try to coerce the position argument in the varargs path of Array#insert, matching behavior changed for https://bugs.ruby-lang.org/issues/13558.
• Also route specific-arity calls one the variable-arity path to their specific-arity forms. This largely eliminates the need for the first change above, but is more consistent with other places we have both specific and variable-arity method bindings.