Replace instant with web-time

[alphaville]

Main Changes

• Swap the cross-platform timer dependency to web-time
• remove instant-specific wasm feature wiring
• update optimizer timing call sites to use web_time::Instant
• keep existing native and wasm timing behavior without stdweb risk
• completely remove wasm

Associated Issues

• Closes Consider swapping to web-time from instant #323

Merge dependencies

• Merge Return best inner solution #388 first
• Merge together with Rust constraints: additional unit tests #396 and release new version

Target versions

• OpEn version 0.11.2

Checklist

• Documentation
• All tests must pass
• Update CHANGELOG(s)
• Update webpage documentation
• Bump versions (in CHANGELOG, Cargo.toml and VERSION)