Add initial complex-number support

- Library type instead of builtin
 - All C complex functions implemented

Partial WIP: Needs more tests for edge cases.

std/math/complex/log.zig

@@ -0,0 +1,26 @@
+const std = @import("../../index.zig");
+const debug = std.debug;
+const math = std.math;
+const cmath = math.complex;
+const Complex = cmath.Complex;
+
+pub fn log(z: var) Complex(@typeOf(z.re)) {
+    const T = @typeOf(z.re);
+    const r = cmath.abs(z);
+    const phi = cmath.arg(z);
+
+    return Complex(T).new(math.ln(r), phi);
+}
+
+const epsilon = 0.0001;
+
+test "complex.clog" {
+    const a = Complex(f32).new(5, 3);
+    const c = log(a);
+
+    const re = c.re;
+    const im = c.im;
+
+    debug.assert(math.approxEq(f32, re, 1.763180, epsilon));
+    debug.assert(math.approxEq(f32, im, 0.540419, epsilon));
+}