lib: add the toHex and toBase utility functions

`toHex` converts the given positive integer to a string of the hexadecimal
representation of that integer. For example:

```
toHex 0 => "0"

toHex 16 => "10"

toHex 250 => "FA"
```

`toBase base i` converts the positive integer `i` to a list of it
digits in the given `base`. For example:

```
toBase 10 123 => [ 1 2 3 ]

toBase 2 6 => [ 1 1 0 ]

toBase 16 250 => [ 15 10 ]
```

lib/tests/misc.nix

@@ -102,6 +102,16 @@ runTests {
     expected = 9;
   };
 
+  testToHex = {
+    expr = toHex 250;
+    expected = "FA";
+  };
+
+  testToBase = {
+    expr = toBase 2 6;
+    expected = [ 1 1 0 ];
+  };
+
 # STRINGS
 
   testConcatMapStrings = {