Replace JS snippet with WASI call for random byte generation

[palas]

Changelog

- description: |
    Use WASI call to generate random bytes instead of JS snippet
  type:
  - compatible
  projects:
  - cardano-wasm

Context

We want to produce a WASM module that doesn't depend on JavaScript function calls to work, only in WASI, so that it can be used from other languages. This PR modifies the call used to generate random ByteStrings so that it doesn't depend on JavaScript but uses the random_get primitive from WASI. Which is described as producing high quality randomness, and is actually implemented in terms of crypto.getRandomValues in the shim we use.

How to trust this PR

It is tricky, because it is really sensitive code, and the code is quite simple, but it does rely on the WASI implementation underneath. I will do some testing and you can see the implementation in the shim we are currently using for JS here: https://github.com/bjorn3/browser_wasi_shim/blob/8dee2473332e540b3f3a9c59259dd027dc1e6853/src/wasi.ts#L886

Something worrying is It seems it uses Math.random() as fallback, which is not cryptographically secure. But until we can include WASI2, this seems the best primitive.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff