first try at writing an efficient and "correct" exp10

this is a nonstandard function so it's not clear what conditions it
should satisfy. my intent is that it be fast and exact for positive
integral exponents when the result fits in the destination type, and
fast and correctly rounded for small negative integral exponents.
otherwise we aim for at most 1ulp error; it seems to differ from pow
by at most 1ulp and it's often 2-5 times faster than pow.

include/math.h

@@ -387,6 +387,10 @@ float       y1f(float);
 long double y1l(long double);
 float       ynf(int, float);
 long double ynl(int, long double);
+
+double      exp10(double);
+float       exp10f(float);
+long double exp10l(long double);
 #endif
 
 #ifdef __cplusplus