v0.0.8: no more mutability

spec.md

@@ -10,7 +10,6 @@ Most important ones:
 - `List[T]`: (linked-) list of `T`
 - `Char`: unicode codepoint
 - `Unit`: nothing
-- `Cell[T]`: mutable cell of `T`
 
 Advanced (uncommon) types:
 - `Int8`, `Int16`, ...
@@ -42,14 +41,6 @@ let name = "Max"
 let passw = "1234"
 ```
 
-## Mutability only via `ref`
-```
-let value = Cell.from(1)
-print(Num.to_str(!value))     # !cell "observes" the cell
-value := 2                    # `:=` mutates the cell
-print(Num.to_str(!value))
-```
-
 ## no confusing function or operator overloading
 all operators:
 - `Num + Num` (has overloads for fixed width number types)
@@ -60,8 +51,6 @@ all operators:
 - `List[t] ++ List[t]`: list concatenation
 - `value :: t` (explicitly specify type of value, useful for down-casting structs, or just code readability; does not perform casting)
 - `list[index]`
-- `!cell`: "observe" a mutable value
-- `cell := value`: mutate a mutable value
 
 ## non-nominal struct types
 ```