Implement unread_byte for byte_buf

[catamorphism]

In the impl of reader for byte_buf, in core::io, unread_byte currently fails with an "implemented" message.

[eholk]

This should be pretty easy to fix. I think it basically just needs to call vec::unshift on the underlying vector.

[catamorphism]

n.b. byte_buf is now BytesReader.

[catamorphism]

I tried to do this, but I got confused. Since BytesReader wraps an immutable vector, I don't see a safe way to implement unread_byte. Do we even need it? I don't see it used anywhere.

[erickt]

I'm fine with removing it. How about we remove it, and make an issue to add a Writer wrapper that adds support for unreading bytes/chars?

[catamorphism]

Sounds like a plan!

[erickt]

Here it is: #4031.

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.debug-assertions := True
configure: rust.overflow-checks := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
Attempting with retry: make prepare
---
    Checking tracing-error v0.2.0
error: lifetime parameter `'tcx` never used
  --> src/tools/miri/src/shims/unix/linux/fd/epoll.rs:39:12
   |
39 |     fn dup<'tcx>(&mut self) -> io::Result<Box<dyn FileDescriptor>> {
   |           -^^^^- help: elide the unused lifetime
   |
   = note: `-D unused-lifetimes` implied by `-D warnings`
error: lifetime parameter `'tcx` never used
  --> src/tools/miri/src/shims/unix/linux/fd/event.rs:24:12
   |
   |
24 |     fn dup<'tcx>(&mut self) -> io::Result<Box<dyn FileDescriptor>> {
   |           -^^^^- help: elide the unused lifetime
error: lifetime parameter `'tcx` never used
  --> src/tools/miri/src/shims/unix/linux/fd/socketpair.rs:18:12
   |
   |
18 |     fn dup<'tcx>(&mut self) -> io::Result<Box<dyn FileDescriptor>> {
   |           -^^^^- help: elide the unused lifetime
    Checking color-spantrace v0.2.0
    Checking addr2line v0.17.0
    Checking color-eyre v0.6.2
    Checking ui_test v0.5.0