rollup merge of #23668: alexcrichton/io-zero

This commit alters the behavior of the `Read::read_to_end()` method to zero all
memory instead of passing an uninitialized buffer to `read`. This change is
motivated by the [discussion on the internals forum][discuss] where the
conclusion has been that the standard library will not expose uninitialized
memory.

[discuss]: http://internals.rust-lang.org/t/uninitialized-memory/1652

Closes #20314

diff --cc src/libstd/io/mod.rs
Simple merge