Reference implementation for executing mapdag Graphs - a portable interpreter.

AOT-compilation of graphs on the JVM, improving performance by leveraging JVM-specific code generation (via `clojure.core/eval`) and primitive values.

Compared to the reference mapdag interpreter, addresses the following sources of execution overhead:

1. Dynamic argument passing to the `:mapdag.step/compute-fn` (via `clojure.core/apply`). This implementation generates arity-aware code.
2. Map-based navigation. This implementation generates code that navigates the graph through primitive array lookups.
3. Megamorphic JVM call sites (when calling the `:mapdag.step/compute-fn`), impeding JIT optimization. This function eschews this by generating code which defines one function per step, and does static dispatch on the keys (using `clojure.core/case`).