Rename overloaded operators (add underscores)

* add suffix unary operators
* add _! operator
* _._ and _&_ operator overloading
* allow calling function references with _! and access variables with _._
* rename . method operator to !
* various cleaning & improvements

Documentation will follow at some point.

test/tests/binop assignement.lua

@@ -0,0 +1,22 @@
+local _={}
+_[9]={}
+_[8]={}
+_[7]={tags=_[9],text="3"}
+_[6]={tags=_[8],text="1"}
+_[5]={_[7]}
+_[4]={_[6]}
+_[3]={"return"}
+_[2]={"text",_[5]}
+_[1]={"text",_[4]}
+return {_[1],_[2],_[3]}
+--[[
+{ "text", { {
+      tags = {},
+      text = "1"
+    } } }
+{ "text", { {
+      tags = {},
+      text = "3"
+    } } }
+{ "return" }
+]]--