Rollup merge of #118222 - the8472:copy-use-vec-write, r=m-ou-se

unify read_to_end and io::copy impls for reading into a Vec

This ports over the initial probe (to avoid allocation) and the dynamic read sizing from the io::copy specialization to the `default_read_to_end` implementation which already had its own optimizations for different cases.

I think it should be a best-of-both now.

suggested by `@a1phyr` in rust-lang/rust#117576 (comment)