Unnecessary buffer initialization in ByteQueue::push

[sporksmith]

ByteQueue::push uses the standard library's Read::read_to_end. To avoid making uninitialized data available to the Read implementation, it currently initializes the buffer before passing it to read.

It looks like there's currently an internal / nightly API for marking a reader as promising not to read the buffer it's passed, in which case the initialization will be skipped.

• https://doc.rust-lang.org/src/std/io/mod.rs.html#391
• https://doc.rust-lang.org/std/io/trait.Read.html#method.initializer

In the meantime we may want to replace our usage of read_to_end with something that doesn't pre-initialize.

Example from a perf report, in a benchmark reading and writing 64 KB buffers via a pipe:

-   18.48%     0.26%             4  worker-0  shadow  [.] shadow_rs::utility::byte_queue::ByteQueue::push
   - 18.22% shadow_rs::utility::byte_queue::ByteQueue::push
      - 16.16% <std::io::Take<T> as std::io::Read>::read_to_end
         + 8.80% <shadow_rs::host::memory_manager::MemoryReaderCursor as std::io::Read>::read
           6.47% __memset_avx2_erms 

We spend almost as much time zeroing the buffer (__memset_avx2_erms) as copying the data (read)

[sporksmith]

Btw it looks like the initializer method is unlikely to be stabilized. It's been superseded by the (merged, but unimplemented) ReadBuf RFC

[robgjansen]

We decided this doesn't really fit in any milestone currently, and we are waiting for improvements in the Rust API before we can make progress.