STL rework

- fixed lots of bugs
- overlay libraries work correctly and reliably
- the STL is an overlay library
- examples updated

examples/calculator/main.orc

@@ -1,16 +1,15 @@
-import std::(parse_float, to_string)
-import std::(readline, print)
+import std::(proc::*, to_float, to_string, io::(readline, print))
 
 export main := do{
   cps print "left operand: ";
   cps data = readline;
-  let a = parse_float data;
+  let a = to_float data;
   cps print "operator: ";
   cps op = readline;
   cps print ("you selected \"" ++ op ++ "\"\n");
   cps print "right operand: ";
   cps data = readline;
-  let b = parse_float data;
+  let b = to_float data;
   let result = (
     if op == "+" then a + b
     else if op == "-" then a - b