Added simple assignment, read more

I was coding earlier this week, but my brain was so foggy I ended up not
knowing what I was doing. After a few days break, I've cleaned up the
mess, which took hours.

Changes:
* Variables can be assigned
* Added new value types as placeholders
* Added 'compare' and 'assign' to the AST
* Added duplicate opcode
* Added functions to copy and free values
* Max name length is 255 chars
* Compound assigns are squeezed into one word

To be completed:

* Tests for this commit's changes
* Compound assignments
* Variable access

tests/integrations/test_variables_and_scopes.toy

@@ -4,3 +4,19 @@ var answer = 42;
 //declare a variable without an initial value
 var empty;
 
+//assign a previously existing variable
+answer = 6 * 9;
+
+
+/* TODO: implement compound assignments
+answer += 5;
+
+answer -= 5;
+
+answer *= 9;
+
+answer /= 2;
+
+answer %= 10;
+
+*/