jank now has its own custom IR

The jank Clojure dialect compiler now has a custom SSA-based intermediate representation (IR) operating at Clojure's semantic level, enabling high-level optimizations that LLVM IR cannot perform. The post walks through a series of optimizations applied to a recursive fibonacci benchmark: arithmetic inlining (5522ms → 2309ms), eliminating redundant IR instructions (→ 2247ms), fixing nil access overhead (→ 1400ms), implementing pointer tagging for 63-bit integers to eliminate heap allocations (→ 282ms), and forcing inlining of arithmetic functions via C++ attributes (→ 114ms). The final result is jank running nearly 2x faster than Clojure JVM for this benchmark, down from being 27x slower.