v0.27.2

Measurements only (no code change): Round 41 — an opt-in FMA mode for the amd64 kernels, measured. As accurate as the default (relative RMS error 0.90–1.04× the default's, ≤ 1.1e-15, against numpy and long-double scipy on the exact input bytes); complex 256 1.07–1.08× faster, RFFT 4096 on Zen 3 1.116 → 1.044 × FFTW, rows within 5% of FFTW Zen 3 18 → 20, Cascade Lake 16 → 18; complex 1024 unchanged; a few rows up to 2.2% slower. The code (branch fma-opt-in-r41, #77) is not merged yet: its FFT kernel generators are first moving into go-asmgen. #76