pw_containers: Improve templated Vector::insert

This CL reworks the templated overload of `Vector::insert` that takes
two iterators to only match actual iterators. Previously, the template
parameter `Iterator` could be anything, and calls meant for
`insert(..., size_type, const T&)`, e.g. `insert(..., 3, 3)`, would
instead resolve to the templated method and fail to compile.

Change-Id: I3821417b055ab65328df3e6bebf627c760bf28f1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/149031
Reviewed-by: Taylor Cramer <cramertj@google.com>
Commit-Queue: Aaron Green <aarongreen@google.com>