feat: improve borrow checker — reinitialization, assignment move, return move

- Add isOwn field to Symbol for tracking own T declarations
- Reinitialization after move: assigning to a moved variable removes it from movedVars
- Move tracking in let/var initialization: var b: own Box = a moves a
- Move tracking in assignment: x = y moves y if y is own T
- Move tracking in return: return x moves x if x is own T
- Expand borrow_test.nim to 10 tests (all passing)

compiler/bootstrap/scope.nim

@@ -16,6 +16,7 @@ type
     decl*: Decl        ## optional back-reference to AST decl
     isMutable*: bool
     isPublic*: bool
+    isOwn*: bool       ## true if declared as own T
 
   Scope* = ref object
     parent*: Scope