Improve pointer documentation of the applet API

[ia0]

This also fixes some soundness issues in the Rust prelude.

[ia0-review]

Seems like there's a bug when running the store test applet on the host runner.

[ia0]

The applet is trapping because the allocator now returns a null pointer on zero size instead of allocating (which was wasteful but correct). Let's specify somewhere (api-desc documentation?) the common safety invariants regarding pointer-related functions (exported alloc and all imported functions mentioning pointers including function pointers).

[ia0]

@not-an-aardvark Feel free to review if you got time (you can filter out the Cargo.lock files which are just noise, as well as the Cargo.toml and CHANGELOG.md files). The main file is crates/api-desc/src/api.md which describes the general invariant.