Get the max RSS from getrusage(2)

[eregon]

• This captures the maximum RSS usage over the process lifetime so far.
• Notably this is more stable than just getting RSS at the end.

This is using Fiddle as that works on CRuby, JRuby and TruffleRuby (FFI is not shipped with CRuby, JRuby does not support C extensions and loading a C extension for this would be heavy at least on TruffleRuby).

This max RSS stat is a lot more stable than RSS and tracks the maximum memory usage, for example here are some runs on TruffleRuby:

# 1
RSS: 1065.4MiB
MAXRSS: 1631.9MiB
# 2
RSS: 810.3MiB
MAXRSS: 1603.3MiB
# 3
RSS: 1210.5MiB
MAXRSS: 1588.4MiB

On CRuby they are closer in these measurements, but RSS might still be brittle as e.g. a GC might happen "by luck" during the last iteration:

# 1
RSS: 558.5MiB
MAXRSS: 565.2MiB
# 2
RSS: 559.6MiB
MAXRSS: 566.3MiB

I also chatted about this with @noahgibbs a while ago, but finally got some clear numbers to show the value of it.

[eregon]

I am unsure if this maxrss stat should replace the existing rss one, so in doubt I kept both. I think it might make sense to replace the RSS one since it's so unstable on non-CRuby, and potentially brittle on CRuby (+ might not account for the JIT overhead correctly).

[noahgibbs]

This looks very reasonable to me, especially because by tracking both we can make sure we're not seeing any distressing results or unexpected large disparities. @maximecb, does this look reasonable to you?