Add benchmarks for node operations

[Xuanwo]

Although the performance is not the top thing for serde-env, we need to know how slow we are.

Slow as xx ms is acceptable, but slow as xx s is not allowed.

For the worst case:

:) true | xargs --show-limits
Your environment variables take up 5378 bytes
POSIX upper limit on argument length (this system): 2089726
POSIX smallest allowable upper limit on argument length (all systems): 4096
Maximum length of command we could actually use: 2084348
Size of command buffer we are actually using: 131072
Maximum parallelism (--max-procs must be no greater): 2147483647

We need to make sure serde-env works in that case.