couple more chapters

ch4.md

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+- add text to a string
+- try reassigning the value of a variable to another one, using different types, and see what the compiler says
+- try passing a String instance and/or a Vector to a function which mutates it
+- borrow a non-mutable reference and store it in a var, then print a value made from it (e.g. add one to it)
+- use slicing to get the first word in a multi-word string and print it
+- use assert_eq! to prove what slicing actually does;

ch5.md

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+// create a struct Rectangle, make an instance of it, then calculate the area using a function
+// use dbg! at various points
+// make a new rectangle instance from the first one
+// when does the original rectangle stay valid, when not?
+// add an area method to the Rectangle struct