First native JIT, inline caches, ropes, and a first win over V8

The register VM gets an x86-64 JIT for hot integer functions, OSR loop regions, call-free inline caches, scalar replacement, rope strings and a microtask loop, and beats V8 on loop.js at 26.5 ms against 28.0.

In one day: 59032ebd "native x86-64 JIT for hot integer functions (4.1x where it fires)"; b882eea5 "OSR loop-region JIT (double/SSE2)"; 050fe88d "call-free inline cache for GetProp/SetProp — object.js 14ms → 3ms"; bca59c77 scalar replacement, "object 31ms → 8ms"; e6e63a0a "rope strings + cached length/index — kill the O(n²) in string concat & scan"; 0db0d5af "Remove the old JS engines; zipp js now runs zipp-vm"; a5cf5a3e promises and a microtask event loop; and d7d9d5e0, "cheap 2^53 guard (range trick) — BEAT V8 on loop.js (26.5 vs 28.0ms)".