* Download sprout parameters in-memory
* Add download_sapling_parameters and deprecate download_parameters
* This avoids confusion between sprout and sapling downloads,
while maintaining backward compatibility.
* Download a single file, rather than parts
* This is more efficient, because TCP adjusts its transfer speed
in the first ~20 seconds of each new connection.
* Only download files if needed, but always check the hashes
* Allow the caller to specify a response timeout
* Stream downloads from server to disk
* Refactor file loads to use the same verifying function as downloads
* Check file sizes to help debug parameter load failures
* Remove downloaded files on error (but leave existing files alone)
* Add a sprout and sapling download example
* Move the download Read impl into its own module
* Derive standard traits on SaplingParameterPaths
* Require features for the load parameters method