Major change to wasm_js backend

Now, when the wasm_js feature is enabled, the wasm_js backend will be used
by default. Users of wasm32-unknown-unknown targeting JavaScript environments
like the Web and Node.js will no longer need to specify:

--cfg getrandom_backend="wasm_js"

in RUSTFLAGS for the crate to compile. They can now simple enable a feature.

Note: this should not affect non-JS users of the wasm32-unknown-unknown
target. Using --cfg getrandom_backend will still override the source of
randomness even if the wasm_js feature is enabled. This includes
--cfg getrandom_backend=custom and --cfg getrandom_backend=unsupported.

For more information, see the discussions in #671, #675, and #730.

Added

• unsupported opt-in backend #667
• windows_legacy opt-in backend #724

Changed

• Implement Memory Sanitizer unpoisoning more precisely #678
• Relax MSRV for the linux_raw opt-in backend on ARM targets #688
• Use getrandom syscall on all RISC-V Linux targets #699
• Replaced wasi dependency with wasip2 #721
• Enable wasm_js backend by default if the wasm_js feature is enabled #730

Removed

• Unstable rustc-dep-of-std crate feature #694