added todo

todo.txt

@@ -0,0 +1,23 @@
+Errors:
+- Not supported
+- Only * is supported
+- Expected * in *, but got *
+- Divergent type in (construct), (location 1), (location 2)
+- Unknown field
+- Unbounded variable
+- Different variable
+- Different domain
+- * is not subclass of *
+- Type not equal
+- Incorrect number of parameters
+
+Symbol Resolution:
+- Find class references, load them all in TopLevelContext.
+- Find unbounded identifiers/functions in functions.
+- `inspect.getmodule(x)` every unknown `x` if not primitive/list of primitives,
+  check their files recursively by getting their file path, cache with
+  (normalized) path as key.
+
+TopLevelContext/InferenceContext:
+- Restrict visibility by user defined function.
+