Add no_std support

[daxpedda]

• Through no_std support, we now also support wasm32v1-none.

• Introduced a new std crate feature that is enabled by default. Without it #[no_std] is enabled, but only on Web! This allows Web target to build without std, which is now supported by wasm-bindgen as well.

  Additionally, various changes had to be done to support no_std:

  • no_std does not support thread_local!, we use once_cell to polyfill this gap. once_cell is not a new dependency, it is already a dependency of wasm-bindgen.

    • With feature = "std", we use thread_local! as before.
    • Without std and without target_feature = "atomics", we use a static mut with once_cell::unsync::Lazy.
    • Without std and with atomics, we use #[thread_local] with once_cell::unsync::Lazy.

  • Some f64 instructions are not available on no_std and had to be polyfilled. For this code from libm was copied. Which is used by std as well.

  • SystemTimeError now only implements Error with feature = "std".

  • no_std testing requires to refrain from using the default test harness. The problem was that native tests still needed to use the default test harness. To solve this, integration tests were removed from root crate and two workspace members added, that manually define all integration tests as test targets. The tests-web crate has harness = false on all tests, while tests-native functions regularly. This allow us to use the default test harness for native tests while disabling it for Web.

    Additionally, every test target requires the run crate feature, which are enabled by default depending on the target by the root crate. This way regular testing can function correctly for each target as long as --all-features is not used. E.g. cargo test --workspace and cargo test --workspace --target wasm32-unknown-unknown will work correctly.

    The tests-web library is used to implement the panic_handler, global_allocator and critical_section. It is always imported to reduce code duplication in all tests.

  • Used serde-json-core to cover tests with no_std as well.

  • All links to std documentation had to be supplemented with manual link when std is not present.

• Improvements to CI:

  • Refactored all matrices for simplification and covering --no-default-features.
  • Split regular and minimal versions building off tests.
  • Split doctests from regular tests.
  • Update Rust toolchain when testing cargo publish.
  • Test coverage improvements:
    • The new wasm-bindgen update allows us to refrain from having to pass cfg flags to the wasm-bindgen proc-macros.
    • Use Rust llvm-tools instead of the official LLVM package.
    • Remove invalid script tag from coverage report.
    • Retain coverage artifact instead of limiting it to one day.
    • Refactor large environment variables into global level ones.

• Small fixes that were stumbled upon:

  • Expose web_time::web with cfg(docsrs) on native as well.
  • Serialize and Deserialize implementation are now marked with doc(cfg(feature = "serde")).
  • Std docs.rs link unnecessarily including stable.
  • Recommendation for -Ctarget-feature=+nontrapping-fptoint was moved from the top-level to the "Usage" section.
  • The minimal version of Serde is now fully specified as v1.0.0.
  • Fix some typos in internal documentation.

no_std support for serde_test: serde-deprecated/test#36
no_std support for getrandom: rust-random/getrandom#541.
no_std Wasm f64 instructions: rust-lang/stdarch#1677