add: slice internals backing array and slice header

16-slices/questions/5-slice-header.md

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+# Slice Header Quiz
+
+## What is a slice header?
+1. The first element of a slice value
+2. The first element of the backing array
+3. A tiny data structure that describes all or some part of a backing array *CORRECT*
+4. A data structure that contains the elements of a slice
+
+> **3:** Yes! It's just a tiny data structure with three numeric fields.
+> 
+> **4:** A slice doesn't contain any elements on its own.
+
+
+## What are the fields of a slice value?
+1. Pointer, length, and capacity *CORRECT*
+2. Length and capacity
+3. Only a pointer
+
+
+## Which slice value does the following slice header describe?
+SLICE HEADER:
++ Pointer : 100th
++ Length  : 5
++ Capacity: 10
+
+Assume that the backing array is this one:
+```go
+var array [10]string
+```
+1. array[5:]
+2. array[:5] *CORRECT*
+3. array[3:]
+4. array[100:]
+
+> **1**: This slice's capacity is 5, it can only see the elements beginning with the 6th element.
+> 
+> **2**: That's right. `array[:5]` returns a slice with the first 5 elements of the `array` (len is 5), but there are 5 more elements in the backing array of that slice, so in total its capacity is 10.
+> 
+> **3**: This slice's capacity is 7, it can only see the elements beginning with the 4th element.
+> 
+> **4**: This is an error. The backing array doesn't have 100 elements.
+> 
+
+
+## Which one is the slice header of the following slice?
+```go
+var tasks []string
+```
+1. Pointer: 0, Length: 0, Capacity: 0 *CORRECT*
+2. Pointer: 10, Length: 5, Capacity: 10
+3. Pointer: 0, Length: 1, Capacity: 1
+
+> **1:** A nil slice doesn't have backing array, so all the fields are equal to zero.
+
+
+## What is the total memory usage of this code?
+```go
+var array [1000]int64
+
+array2 := array
+slice := array2[:]
+```
+
+1. 1024 bytes
+2. 2024 bytes
+3. 3000 bytes
+4. 16024 bytes *CORRECT*
+
+> **4:** `array` is 1000 x int64 (8 bytes) = 8000 bytes. Assigning an array copies all its elements, so `array2` adds additional 8000 bytes. A slice doesn't store anything on its own. Here, it's being created from array2, so it doesn't allocate a backing array as well. A slice header's size is 24 bytes. So in total: This program allocates 16024 bytes.
+
+
+## What value does this code pass to the sort.Ints function?
+```go
+nums := []int{9, 7, 5, 3, 1}
+sort.Ints(nums)
+```
+1. [9 7 5 3 1] — All the values of the nums slice
+2. A pointer to the backing array of the nums slice
+3. A pointer, length and capacity as three different arguments
+4. The slice header that is stored in the nums variable *CORRECT*
+
+> **1:** No, a slice value doesn't contain any elements. So it cannot pass the elements.
+> 
+> **2:** Sorry but not only that.
+> 
+> **3:** Nope. Remember, they are packed in a tiny data structure called the ....?
+> 
+> **4:** Yep! A slice value is a slice header (pointer, length and capacity). A slice variable stores the slice header.
+> 
+