perf: use str::make_ascii_lowercase instead of str::to_lowercase

`str::to_lowercase` had been used for the purposes of case-insensitive
comparisons of command names, but `str::make_ascii_lowercase` is
substantially more performant. Using:
    $ redis-benchmark -c 1 -r 128 -t set -n 100000

...with `str::to_lowercase`:
  throughput summary: 6651.59 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
        0.136     0.072     0.135     0.223     0.287     2.807

...with `str::make_ascii_lowercase`:
  throughput summary: 9102.49 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
        0.099     0.064     0.095     0.127     0.191     0.815

While micro-optimizations are usually not appropriate for mini-redis,
this pattern of case-insensitive string comparisons is common, and
using `str::to_lowercase` sets a bad example.

Author: jswrenn

src/cmd/mod.rs

@@ -53,8 +53,10 @@ impl Command {
 
         // All redis commands begin with the command name as a string. The name
         // is read and converted to lower cases in order to do case sensitive
-        // matching.
-        let command_name = parse.next_string()?.to_lowercase();
+        // matching. Doing this in-place with `str::make_ascii_lowercase` is
+        // substantially more performant than `str::to_lowercase`.
+        let mut command_name = parse.next_string()?;
+        command_name.make_ascii_lowercase();
 
         // Match the command name, delegating the rest of the parsing to the
         // specific command.