Added to_string() and to_number() functions as built-ins.

examples/builtins_conversions.fun

@@ -0,0 +1,37 @@
+#!/usr/bin/env fun
+
+// Conversions and length built-ins demo
+
+// len on array and string
+print(len([1, 2, 3]))          // -> 3
+print(len("hello"))            // -> 5
+
+// to_number with various string inputs
+print(to_number("42"))         // -> 42
+print(to_number("   -7  "))    // -> -7
+print(to_number("12a"))        // non-numeric suffix -> 0
+print(to_number(""))           // empty -> 0
+print(to_number("  "))         // spaces -> 0
+
+// to_string on numbers and arrays
+print(to_string(42))           // -> "42" (printed as 42)
+
+arr = [1, 2]
+s = to_string(arr)             // -> "[array n=2]"
+print(s)
+
+// combine to_string with concatenation
+print("val=" + to_string(99))  // -> "val=99"
+
+/* Expected output:
+3
+5
+42
+-7
+0
+0
+0
+42
+[array n=2]
+val=99
+*/