Support platforms without a timer

I've dabbled recently in seeing how hard it would be to compile rustfmt to wasm
and then run it in a web browser, and it turns out that it's [not too
hard][wasm]! In addition to patching a few dependencies which already have a
number of patches out rustfmt also needed some modifications to get it to work,
namely avoiding the usage of `Instant::now()` on the "happy path" which doesn't
work on wasm (it just panics).

This commit is an attempt to add a support for this by avoiding using
`Instant::now()` on the wasm target, but panicking if the actual time elapsed is
requested (which doesn't happen unless verbosely logging I believe).

[wasm]: https://alexcrichton.github.io/rustfmt-wasm/

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