Make insert_many fallback linear with empty and single fast paths

[charliermarsh]

Summary

When an iterator produces more elements than its lower size hint, insert_many shifts the existing tail once for every extra element. We now append the excess elements and rotate them into position once, avoiding that repeated movement. Empty and single-element inputs have fast paths, and a guard puts successfully appended elements in place if iteration panics.

Across five native Rust benchmark pairs on x86_64 Linux, inserting 1,024 elements before a 1,024-element tail dropped from about 90 µs to 0.65 µs. Empty and single insertions stayed around 14 ns and 21 ns, respectively; a short insertion with a weak positive size hint was about 6% slower. The large gain comes from avoiding repeated tail movement in that workload.

The added native benchmarks were removed in a follow-up commit and remain in the PR history.