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

source/toy_opcodes.h

@@ -7,6 +7,8 @@ typedef enum Toy_OpcodeType {
 	TOY_OPCODE_ASSIGN,
 	TOY_OPCODE_ACCESS,
 
+	TOY_OPCODE_DUPLICATE, //duplicate the top of the stack
+
 	//arithmetic instructions
 	TOY_OPCODE_ADD,
 	TOY_OPCODE_SUBTRACT,