WASM: Expand benchmark testing for basic demo

[mbestavros]

PR #18 implemented performance benchmarking on the basic Wasmtime demo. We'd like to expand these benchmarks to give a better idea of performance inside the Wasmtime VM.

Specifically, we'd like to make the native Rust number addition test have random inputs, and better replicate that test inside the Wasmtime VM without needing to cross the VM threshold to invoke function calls.