use short hyphenated names for benchmarks

[jkarneges]

This ensures uniform output (no long names causing some results to be pushed to the next line) and removes the need for quotes when filtering (cargo bench rc-drop instead of cargo bench "rc drop").

% cargo bench --bench client 2>&1 | grep time:              
client-req-1w-x10       time:   [1.0755 ms 1.1279 ms 1.1881 ms]
client-stream-1w-x10    time:   [1.7949 ms 1.8457 ms 1.9040 ms]
client-req-2w-x10       time:   [1.0841 ms 1.1416 ms 1.1986 ms]
client-stream-2w-x10    time:   [2.0524 ms 2.1434 ms 2.2477 ms]