Change alias shape inference to proposal of RFC#4 (#2685)

* Change alias shape inference to proposal of RFC#4

* Remove commented code

* Fix typo

* Change comment to be more informative

* Make match statement to lookup in table

* Remove resolved question

https://github.com/nushell/nushell/pull/2685#discussion_r509832054

* Pick ...or_insert_dependency functions into pieces

Previously there was get_shape_of_expr_or_insert dependency, now there is
get_shape_of_expr and get_shape_of_expr_or_insert_dependency

2 new functions have been added: get_result_shape_of_math_expr and
get_result_shape_of_math_expr_or_insert_dependency

* Remove flattening of deep binary expressions

Previously deep binary expressions have been flattened through the insertion of
fake vars. This logic was quite complicated. Now if a variable depends on the
result shape of a binary expression and the result shape can't be computed,
the variable simply depends on the whole binary.

* Change Expression::Variable(Variable::It(...)) to Expression::Variable(...)

* Simplify get_result_shapes_in_math_expr

* Simplify infer_shapes_in_binary_expr

* Clarify comment

* Clarify comment

* Fix clippy lint

* Move check for real var into checked_insert

* Remove comment

* Rename var

crates/nu-protocol/src/signature.rs

@@ -26,7 +26,7 @@ impl NamedType {
 }
 
 /// The type of positional arguments
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub enum PositionalType {
     /// A mandatory positional argument with the expected shape of the value
     Mandatory(String, SyntaxShape),