Hot loops get compiled

This is the start of a long-promised speed-up: Floati now has a just-in-time compiler for the hottest JavaScript. When a page runs the same loop over and over (counting, arithmetic, crunching numbers), Floati notices it is hot and compiles it into a faster form that keeps the loop's variables in a fast, fixed place instead of looking each one up by name on every single pass, which is the real cost in a plain interpreter. In our benchmarks number-heavy loops now run about two and a half to three times faster. The interpreter is always still there underneath: if a loop does anything the fast path does not handle, Floati quietly steps back to it, so behaviour never changes, only speed. Pages whose work is mostly calling functions or touching the page are unchanged for now; those come next.