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

.notes/read-opcode-layout.txt

@@ -0,0 +1,28 @@
+TOY_OPCODE_DECLARE:
+	{Declare, type, length, 0} ; {emitString()}
+
+# write value
+TOY_OPCODE_READ:
+	null   -> {Read, type, 0, 0}
+	bool   -> {Read, type, value, 0}
+	int    -> {Read, type, 0, 0} ; {value}
+	float  -> {Read, type, 0, 0} ; {value}
+	string -> {Read, type, leaf, 0} ; {emitString()}
+
+# write assignment
+TOY_AST_FLAG_ASSIGN:
+	{Read, type(string), name, length} ;
+	{emitString()} ;
+	{writeCode()} ;
+	{Assign, 0, 0, 0} ;
+
+TOY_AST_FLAG_ADD_ASSIGN:
+	{Read, type(string), name, length} ;
+	{emitString()} ;
+	{Duplicate, 0, 0, 0} ;
+	{writeCode()} ;
+	{Assign, 0, 0, 0}
+	{Add, Assign, 0, 0} ;
+
+//subtract, multiply, divide, modulo all mimic add
+