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

makefile

@@ -26,7 +26,10 @@ repl: source
 
 #various kinds of available tests
 .PHONY: tests
-tests: clean test-cases test-integrations test-benchmarks
+tests: clean test-cases
+
+.PHONY: test-all
+test-all: clean test-cases test-integrations test-benchmarks
 
 .PHONY: test-cases
 test-cases: