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)

_test_borrow2/bux.toml

@@ -0,0 +1 @@
+[Package]

_test_borrow2/src/Main.bux

@@ -0,0 +1,38 @@
+import Std::Io::{PrintLine};
+
+@[Checked]
+func Take(x: own String) {
+    PrintLine(x);
+}
+
+@[Checked]
+func UseAfterMoveReinit() {
+    var s: own String = "hello";
+    Take(s);
+    /* s is moved here */
+    /* PrintLine(s); */ /* would error */
+    s = "reinitialized";
+    /* s is valid again after reinitialization */
+    PrintLine(s);
+}
+
+@[Checked]
+func MoveInAssign() {
+    var a: own String = "A";
+    var b: own String = a;  /* a is moved to b */
+    PrintLine(b);
+    /* PrintLine(a); */ /* would error */
+}
+
+@[Checked]
+func MoveInReturn() -> own String {
+    var x: own String = "returned";
+    return x;  /* x is moved */
+}
+
+func Main() -> int {
+    UseAfterMoveReinit();
+    MoveInAssign();
+    PrintLine(MoveInReturn());
+    return 0;
+}