core; num; add exp & log2

I've had a couple use cases in time that the code is significantly clearer with these, and makes porting less error prone

doc/site/modules/core/num.markdown

@@ -96,7 +96,15 @@ Whether the number is [not a number](http://en.wikipedia.org/wiki/NaN). This is
 
 ### **log**
 
-The natural logarithm of the number.
+The natural logarithm of the number. Returns `nan` if the base is negative.
+
+### **log2**
+
+The binary (base-2) logarithm of the number. Returns `nan` if the base is negative.
+
+### **exp**
+
+The exponential `e` (Euler’s number) raised to the number. This: `eⁿ`. 
 
 ### **pow**(power)
 

src/vm/wren_core.c

@@ -652,6 +652,8 @@ DEF_NUM_FN(sin,     sin)
 DEF_NUM_FN(sqrt,    sqrt)
 DEF_NUM_FN(tan,     tan)
 DEF_NUM_FN(log,     log)
+DEF_NUM_FN(log2,    log2)
+DEF_NUM_FN(exp,     exp)
 
 DEF_PRIMITIVE(num_mod)
 {
@@ -1299,6 +1301,8 @@ void wrenInitializeCore(WrenVM* vm)
   PRIMITIVE(vm->numClass, "sqrt", num_sqrt);
   PRIMITIVE(vm->numClass, "tan", num_tan);
   PRIMITIVE(vm->numClass, "log", num_log);
+  PRIMITIVE(vm->numClass, "log2", num_log2);
+  PRIMITIVE(vm->numClass, "exp", num_exp);
   PRIMITIVE(vm->numClass, "%(_)", num_mod);
   PRIMITIVE(vm->numClass, "~", num_bitwiseNot);
   PRIMITIVE(vm->numClass, "..(_)", num_dotDot);

test/core/number/exp.wren

@@ -0,0 +1,3 @@
+System.print(5.exp)        // expect: 148.41315910258
+System.print(10.exp)       // expect: 22026.465794807
+System.print((-1).exp)     // expect: 0.36787944117144

test/core/number/log2.wren

@@ -0,0 +1,4 @@
+System.print(1024.log2)    // expect: 10
+System.print(2048.log2)    // expect: 11
+System.print(100.log2)     // expect: 6.6438561897747
+System.print((-1).log2)    // expect: nan