Optimize creation of buffered readers/writers

I was benchmarking rust-http recently, and I saw that 50% of its time was spent
creating buffered readers/writers. Albeit rust-http wasn't using
std::rt::io::buffered, but the same idea applies here. It's much cheaper to
malloc a large region and not initialize it than to set it all to 0. Buffered
readers/writers never use uninitialized data, and their internal buffers are
encapsulated, so any usage of uninitialized slots are an implementation bug in
the readers/writers.
auto merge of #10424 : alexcrichton/rust/optimize-buffered, r=brson

I was benchmarking rust-http recently, and I saw that 50% of its time was spent
creating buffered readers/writers. Albeit rust-http wasn't using
std::rt::io::buffered, but the same idea applies here. It's much cheaper to
malloc a large region and not initialize it than to set it all to 0. Buffered
readers/writers never use uninitialized data, and their internal buffers are
encapsulated, so any usage of uninitialized slots are an implementation bug in
the readers/writers.