wasm-mutate: add export mutator

[GtechGovind]

Summary

• add a wasm-mutate mutator that exports a randomly selected existing function, table, memory, global, or tag
• preserve existing exports and create the export section at the correct position when absent
• account for local tag sections so tags are available to the mutator
• disable the mutation when preserving semantics or reducing size

Part of #415.

Testing

• cargo test -p wasm-mutate
• cargo clippy -p wasm-mutate --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

[alexcrichton]

Thanks @GtechGovind! FWIW while these PRs are in draft reviewers aren't assigned and typically don't look too closely. When you're ready feel free to take them out of draft and a reviewer will be assigned.