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

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