Deflates the *nva*, which has the *nvlen* name/value pairs, into
the *buf* of length *buflen*.

If *buf* is not large enough to store the deflated header block,
this function fails with
:enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`.  The caller
should use `nghttp2_hd_deflate_bound()` to know the upper bound of
buffer size required to deflate given header name/value pairs.

Once this function fails, subsequent call of this function always
returns :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`.

After this function returns, it is safe to delete the *nva*.

This function returns the number of bytes written to *buf* if it
succeeds, or one of the following negative error codes:

:enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
    Out of memory.
:enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`
    Deflation process has failed.
:enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`
    The provided *buflen* size is too small to hold the output.