Add exp/log/ln operations

- As requested in #18

main_test.go

@@ -142,6 +142,10 @@ func TestRPN(t *testing.T) {
 		{input: "rad 3 sqr atan", want: big().Quo(ctx.Pi(big()), bigUint(3))},          // Pi / 3
 		{input: "deg 1 180 * PI / atan", want: big().Quo(ctx.Pi(big()), bigUint(4))},   // Pi / 4
 
+		// Log functions
+		{input: "c E ln", want: bigUint(1)},
+		{input: "c 1000 log", want: bigUint(3)},
+
 		// Bitwise operations and base input modes.
 		{input: "0x00ff 0xff00 or", want: bigUint(0xffff)},
 		{input: "0x0ff0 and", want: bigUint(0x0ff0)},
@@ -187,7 +191,6 @@ func TestRPN(t *testing.T) {
 			}
 			got := decimal.WithPrecision(precision).Set(stack.top())
 			want := decimal.WithPrecision(precision).Set(tt.want)
-
 			if got.CmpTotal(want) != 0 {
 				t.Fatalf("diff: input: %s, want: %s, got: %s", tt.input, want, got)
 			}

operations.go

@@ -181,7 +181,7 @@ func newOpsType(ctx decimal.Context, stack *stackType) *opsType {
 			return []*decimal.Big{bigUint(z)}, 2, nil
 		}},
 		"",
-		"BOLD:Trigonometric Operations",
+		"BOLD:Trigonometric and Log Operations",
 		ophandler{"sin", "Sine of x", 1, func(a []*decimal.Big) ([]*decimal.Big, int, error) {
 			z := ctx.Sin(big(), radOrDeg(ctx, a[0], ret.degmode))
 			return []*decimal.Big{z}, 1, nil
@@ -206,6 +206,18 @@ func newOpsType(ctx decimal.Context, stack *stackType) *opsType {
 			z := ctx.Atan(big(), radOrDeg(ctx, a[0], ret.degmode))
 			return []*decimal.Big{z}, 1, nil
 		}},
+		ophandler{"exp", "Calculate e ^ x", 1, func(a []*decimal.Big) ([]*decimal.Big, int, error) {
+			z := ctx.Exp(big(), a[0])
+			return []*decimal.Big{z}, 1, nil
+		}},
+		ophandler{"ln", "Natural logarithm of x", 1, func(a []*decimal.Big) ([]*decimal.Big, int, error) {
+			z := ctx.Log(big(), a[0])
+			return []*decimal.Big{z}, 1, nil
+		}},
+		ophandler{"log", "Common logarithm of x", 1, func(a []*decimal.Big) ([]*decimal.Big, int, error) {
+			z := ctx.Log10(big(), a[0])
+			return []*decimal.Big{z}, 1, nil
+		}},
 
 		"",
 		"BOLD:Miscellaneous Operations",