Various refactors

[bens]

Some of these make more sense when recording allocations as well as durations. These changes should make the memory tracking PR much less bulky.

• Move Statistics into its own module and parameterise it by type, so it can hold u64s for nanoseconds and usizes for bytes.
• Compute statistics on demand rather than pre-computing them in Result.
• Add Reading and Readings types, so it can carry more than just nanoseconds without changing most code.
• Try a different approach to formatting using the extensibility methods from std.fmt. This inlines a fair bit of code but it seems easier to inspect and modify than being hidden behind a bunch of functions.
• Add an example for the JSON output.
• Simplify a few examples to be more focused.

If necessary, I can split these into separate PRs.

[hendriknielaender]

Really like this PR 👍. Much more cleaner that we have now Statistics in its own module. Will check it later today - but at first sight it looks good.