vec: Remove the Vec specialization for .extend()
vec: Remove the Vec specialization for .extend()

This now produces as good code (with optimizations) using the TrustedLen
codepath.

GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust