CVE-2021-25900: Buffer overflow in SmallVec::insert_many

The vulnerability explicitly occurs in SmallVec::insert_many as described in all sources. The commit diff shows critical changes to this function's memory handling logic - specifically replacing a direct iterator loop with a two-phase approach (handling size_hint lower bound first, then inserting excess elements safely). The original implementation's unsafe pointer arithmetic and conditional reallocation when 'num_added >= lower_size_bound' was insufficient to prevent buffer overflow when the iterator exceeded its hinted size. The added test_insert_many_overflow in the patch directly validates this failure scenario.