Add solutions to the Strings chapter

Author: BoeingX

src/Strings/ChapterExercises/BuildingFunctions.hs

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+module Strings.ChapterExercises.BuildingFunctions where
+
+-- Question 3
+thirdLetter :: String -> Char
+thirdLetter x = x !! 2
+
+-- Question 4
+letterIndex :: Int -> Char
+letterIndex x = "Curry is awesome!" !! x

src/Strings/ChapterExercises/ReadingSyntax.md

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+# Reading syntax
+
+1. For the following lines of code, read the syntax carefully and
+decide if they are written correctly.
+    a) concat [[1, 2, 3], [4, 5, 6]]
+    b) (++) [1, 2, 3] [4, 5, 6]
+    c) (++) "hello" " world"
+    d) ["hello" ++ " world"]
+    e) "hello" !! 4
+    f) (!!) "hello" 4
+    g) take 4 "lovely"
+    h) take 3 "awesome"
+
+2. We have
+    a) `concat [[1 * 6], [2 * 6], [3 * 6]] = [6, 12, 18]`
+    b) `"rain" ++ drop 2 "elbow" = "rainbow"`
+    c) `10 * head [1, 2, 3] = 10`
+    d) `(take 3 "Julie") ++ (tail "yes") = "Jules"`
+    e) `concat [tail [1, 2, 3], tail [4, 5, 6], tail [7, 8, 9]] = [2,3,5,6,8,9]`

src/Strings/PrintingSimpleStrings/IntermissionExercises.md

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+# Intermission: Exercises
+
+1. In the following code
+```haskell
+Prelude> let x = 5
+Prelude> let y = 7
+Prelude> let z = x * y
+```
+`y` is in scope for `z`.
+
+2. In the following code
+```haskell
+Prelude> let f = 3
+Prelude> let g = 6 * f + h
+```
+`h` is not in scope of `g`.
+
+3. In the following code
+```haskell
+area d = pi * (r * r)
+r = d / 2
+```
+everything is in scope for executing `area`.
+
+4. In
+```haskell
+area d = pi * (r * r)
+    where r = d / 2
+```
+`r` and `d` are in scope for `area`.

src/Strings/TypesOfConcatenationFunctions/IntermissionExercises.md

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+# Intermission: Exercises
+
+1. `(++) [1, 2, 3] [4, 5, 6]`
+
+2. `"<3" ++ " Haskell"`
+
+3. `concat ["<3", " Haskell"]`