learnc: fit the constants in existing C code to data (9.1% smaller JPEGs, +4.9 F1 on WebRTC VAD) [P]

learnc tunes named constants in unmodified C code against a score function you write in C, and writes the same file back with new values. Results, both on held-out data: JPEG, stb_image_write.h (128 quantization-table entries, tuned on 12 Kodak images, tested on the other 12): 9.1% BD-rate at equal