Add more Micro Rust standard library functions

The current Micro Rust standard library is fairly bare-bones. Add more utility functions, their specifications, and proofs of correctness.

The issue names no files, tests, or entry points, so first inspect the Micro Rust standard-library implementation and its existing specifications and proofs. Identify which additional utility functions are appropriate with the project maintainers; done means the selected functions, specifications, and correctness proofs are added and accepted.

Written by the indexing model from the issue text.