Pool::create_buffer cannot be filled through the safe API

Pool::create_buffer(n) allocates n bytes, but there is no safe way to put
anything into them:

let mut buf = request.pool().create_buffer(body.len()).unwrap();
buf.as_bytes_mut()[..body.len()].copy_from_slice(body); // panics

ngx_create_temp_buf leaves pos == last, so Buffer::len() is 0 and
MutableBuffer::as_bytes_mut() hands back an empty slice. That is consistent
with as_bytes_mut meaning "the contents", but it leaves the buffer
unfillable: neither trait exposes the capacity or a way to set the length, and
TemporaryBuffer has no inherent methods besides from_ngx_buf. The only
route is as_ngx_buf_mut() and advancing last by hand, which drops out of
the safe API.

create_buffer_from_str works and covers most response bodies, so this is not
urgent. It does mean create_buffer currently has no safe use, while the
obvious composition above compiles and then takes down the worker.

Something like TemporaryBuffer::append(&mut self, &[u8]) -> usize, or
exposing capacity next to a length setter, would close it.

I'd like to work on this issue.

Environment

• ngx-rust at cda9d8372ccd2e139ef2b03e5806e019e7796187 (ngx 0.5.0)

Start at Pool::create_buffer and the TemporaryBuffer, Buffer::len, and MutableBuffer::as_bytes_mut APIs; compare them with create_buffer_from_str. Check how as_ngx_buf_mut currently permits advancing last, then add a safe way to fill the allocated buffer without an empty-slice panic. Done means create_buffer can be used safely for a byte body and the behavior is covered by a test.