Remove items implement natively in Nasal.

mathlib.c now defines more of these. Remaining items (abs, sgn, min, max)
are likely faster using Nasal than switching to C and back again.

Also add a comment about mod(), clarifying that a native fmod() exists.

Nasal/math.nas

@@ -27,18 +27,13 @@ var avg = func {
     return x;
 }
 
-var pow = func(x, y) { exp(y * ln(x)) }
-
+# note - mathlib defines an fmod function (added after this was written)
+# It uses C-library fmod(), which has different rounding behaviour to
+# this code (eg, fmod(-5, 4) gives -1, whereas this code gives 3)
 var mod = func(n, m) {
     var x = n - m * int(n/m);      # int() truncates to zero, not -Inf
     return x < 0 ? x + abs(m) : x; # ...so must handle negative n's
 }
 
-var asin = func(y) { atan2(y, sqrt(1-y*y)) }
-
-var acos = func(x) { atan2(sqrt(1-x*x), x) }
-
-var tan = func(x) { sin(x) / (cos(x) or die("tangens infinity")) }
-
 var _iln10 = 1/ln(10);
 var log10 = func(x) { ln(x) * _iln10 }