Hand off code to a preinstalled optimized runtime if available

From some rough tests, Watt macro expansion when compiling the runtime in release mode is about 15x faster than when the runtime is compiled in debug mode.

Maybe we can set it up such that users can run something like `cargo install watt-runtime` and then our debug-mode runtime can detect whether that optimized runtime is installed; if it is, then handing off the program to it.

Start by locating the runtime's Watt macro-expansion path and how the debug-mode runtime is launched. Investigate how a cargo-installed optimized runtime could be detected and handed off, then compare release and debug behavior against the reported 15x difference. Done means the approach is defined, implemented, and its performance is measured.

Written by the indexing model from the issue text.