Use web-time instead of unmaintained instant crate.

[airwoodix]

Fixes #538.

The changes are mostly trivial, mainly replacing instant::Duration with web_time::Duration.

I also made some small changes in the relevant doctests, using e.g. Duration::ZERO instead of Duration::new(0, 0) or Duration::from_nanos(x) instead of Duration::new(0, x).

My understanding is that the equivalent of the instant's wasm-bindgen feature is always enabled for web-time, though I didn't check what the CI checks on wasm targets.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.91%. Comparing base (c7673ef) to head (56c94a1).

Files with missing lines	Patch %	Lines
crates/argmin/src/core/executor.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #539      +/-   ##
==========================================
- Coverage   91.92%   91.91%   -0.02%     
==========================================
  Files         177      177              
  Lines       23724    23727       +3     
==========================================
  Hits        21808    21808              
- Misses       1916     1919       +3     

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[airwoodix]

The cargo-deny CI job failure here (multiple occurences) doesn't seem to be directly related to the addition of the web-time dependency (the offending packages are transitively pulled-in by the eframe dependency in the spectator crate).

Would 2bb66df still be ok? The Unicode-3.0 license is OSI-approved and allowed e.g. in the cargo-deny repository.