TypedPython: per-function fastmath opt-in (@compiled(fastmath=True)), not CPython-identical #332

Follow-up of #153 (decision: fastmath stays off by default). Proposal only, not scheduled for implementation: an explicit per-function opt-in such as @compiled(fastmath=True) that allows reassociation and contraction (so float reductions can vectorise), documented as NOT CPython-identical, never on by default, and never applied to a function the user did not mark. Open questions before any implementation: which flags (-ffast-math vs -fassociative-math only), how deopt/identity tests treat such a function, and measurements against numba fastmath=True / Cython -ffast-math once #41 items land.