Feature-flags should not change code behaviour

[thomaseizinger]

Hi!

We are interested in using this crate but we'd like to always use the "fast path" because accuracy is not that important for our usecase.

Unfortunately, we are a library crate (libp2p) and if we activate the feature-flag added in #3, then we will change the behaviour for the entire dependency tree which is not something that we want to force onto our users.

I'd like to suggest to consider the following design:

• Remove the feature-flag
• Offer two functions in the API (names subject to bike-shedding):
  • accurate_memory_stats
  • fast_memory_stats
• Both functions return Option<MemoryStats>
• Users can compose their own fallback behaviour using Option::or

In general, feature flags in cargo should never change code behaviour but only add additional APIs.