Numerically stable sinc function

Implement the function $f(x)=\sin(x)/x$ so that it returns sensible values for every real input.

Special-case small $|x|$ with a Taylor series. At $x=0$ the formula is $0/0$ although the limit is $1$, and very near zero the ratio loses accuracy. Below a small threshold return $1-x^2/6+x^4/120$; otherwise compute $\sin(x)/x$ directly. The function is even, and for large $|x|$ the direct formula is fine.