Adapt tests to work for wasm32-wasi and add wasm32-wasi to CI.

[briansmith]

This is based on PR #1568. See the commit message of the first commit for details. Basically I squashed all the commits together from @JanKaul's PR, removed the changes to build.rs, and the GitHub Actions configuration, and rebased onto main.

For the CI integration, ring-toolchain was updated to include wasmtime for x86-64 Linux. That is used to install wasmtime in CI.

[codecov]

Codecov Report

Merging #1745 (b314eb4) into main (0f8386d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1745   +/-   ##
=======================================
  Coverage   96.10%   96.10%           
=======================================
  Files         137      137           
  Lines       20548    20548           
  Branches      217      217           
=======================================
  Hits        19747    19747           
  Misses        762      762           
  Partials       39       39           

Files	Coverage Δ
src/rand.rs	100.00% <ø> (ø)
tests/aead_tests.rs	98.70% <ø> (ø)
tests/constant_time_tests.rs	100.00% <ø> (ø)
tests/digest_tests.rs	100.00% <ø> (ø)
tests/ed25519_tests.rs	99.35% <ø> (ø)
tests/hkdf_tests.rs	97.10% <ø> (ø)
tests/hmac_tests.rs	96.96% <ø> (ø)
tests/pbkdf2_tests.rs	88.57% <ø> (ø)
tests/rand_tests.rs	100.00% <ø> (ø)
tests/rsa_tests.rs	96.92% <ø> (ø)
... and 1 more

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