This PR assigns this to the "value" member of Variable. I am not sure if our code assumes that this is a constant, so the backend doesn't even visit this variable and uses the value instead. If so, then this won't work, since any modifications to _len will be lost.

Let's add a test for modifying _len.

Here is documentation for Variable:

`symbolic_value` the optional symbolic expression to initialize the variable
(e.g. `2+3+4+x`), this value must be compile time, but it is not necessarily a
constant (e.g., can contain binary operations, other variables, etc.)

`value` the optional constant expression holding the compile time value
(e.g. `5`, or `5.5`), it is a compile time constant.

So value and symbolic_value must be compile time constants. So let's require that for now as well in LPython.

A separate question is if they are set, if the Variable itself has to be a constant or not. I would say if it has to be a constant, then it should have a type Const. Otherwise it can change, which means value is the initial value, but the runtime value can change.

EDIT: This does work on an assignment but makes things more complex as it requires more methods to be moved to CommonVisitor.

Now, this problem gets trickier as Structs don't have a body, and, so AnnAssign which is an Assignment to the struct variables isn't possible with the current design.

One thing that we may do is add a field in the Struct to contain initializers for all it's variable members like the following:

     | StructType(symbol_table symtab, identifier name, identifier* dependencies,
         identifier* members, abi abi, access access, bool is_packed, bool is_abstract,
-        expr? alignment, symbol? parent)
+        expr* initializers, expr? alignment, symbol? parent)

This change is forced by moving visit_Call and visit_List into the common visitor. Both the original mat: Mat = Mat([f64(0.0), f64(0.0)]) and the new mat: Mat = Mat(2.0) is incorrect so I think it's fine for now. The proper fix is to use mat: Mat = Mat(empty((2, 2), dtype=float64)).