fix: update various code snippets for consistency and correctness

Package.swift

@@ -42,8 +42,8 @@ let conceptExerciseTargets: [Target] = conceptExercises.flatMap {
                 .target(name:"\($0.pascalCased)"),
                 .product(name: "Numerics", package: "swift-numerics")
             ], 
-            path:"./exercises/concept/\($0)/Tests",
-            exclude: ["LinuxMain.swift"])
+            path:"./exercises/concept/\($0)/Tests"
+        )
     ]
 }
 

exercises/practice/diamond/.meta/Sources/Diamond/DiamondExample.swift

@@ -2,7 +2,7 @@ struct Diamond {
   static let alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".map { $0 }
 
   static func makeDiamond(letter: Character) -> [String] {
-    guard let index = alphabet.index(of: letter) else {
+    guard let index = alphabet.firstIndex(of: letter) else {
       return []
     }
 

exercises/practice/grade-school/.meta/Sources/GradeSchool/GradeSchoolExample.swift

@@ -2,7 +2,7 @@ class GradeSchool {
   var rosters = [Int: [String]]()
 
    func addStudent(_ name: String, grade: Int) -> Bool {
-    if rosters.values.contains { $0.contains(name) } {
+    if rosters.values.contains(where: { $0.contains(name) }) {
       return false
     }
     rosters[grade, default: []].append(name)

exercises/practice/kindergarten-garden/.meta/Sources/KindergartenGarden/KindergartenGardenExample.swift

@@ -30,8 +30,8 @@ struct Garden {
     let sortedChildren = children.sorted(by: <)
     let lines = diagram.components(separatedBy: CharacterSet.newlines)
     var result = [String: [Plant]]()
-    var line1 = lines[0].map { String($0) }
-    var line2 = lines[1].map { String($0) }
+    let line1 = lines[0].map { String($0) }
+    let line2 = lines[1].map { String($0) }
 
     var index = 0
 

exercises/practice/roman-numerals/.meta/Sources/RomanNumerals/RomanNumeralsExample.swift

@@ -1,6 +1,6 @@
 extension Int {
     func toRomanNumeral() -> String {
-        var arabicToRoman = [1000: "M", 900: "CM", 500: "D", 400: "CD", 100: "C", 90: "XC", 50: "L", 40: "XL", 10: "X", 9: "IX", 5: "V", 4: "IV", 1: "I"]
+        let arabicToRoman = [1000: "M", 900: "CM", 500: "D", 400: "CD", 100: "C", 90: "XC", 50: "L", 40: "XL", 10: "X", 9: "IX", 5: "V", 4: "IV", 1: "I"]
         var i = self
         var s = ""
         for arabic in arabicToRoman.keys.sorted(by: > ) {

exercises/practice/saddle-points/.meta/Sources/SaddlePoints/SaddlePointsExample.swift

@@ -2,7 +2,7 @@ struct SaddlePoints {
   static func saddlePoints(_ matrix: [[Int]]) -> [Position] {
     var saddlePoints = [Position]()
     for (rowIndex, row) in matrix.enumerated() {
-      for (columnIndex, element) in row.enumerated() {
+      for (columnIndex, _) in row.enumerated() {
         if isSaddlePoint(matrix, rowIndex, columnIndex) {
           saddlePoints.append(Position(row: rowIndex + 1, column: columnIndex + 1))
         }

exercises/practice/sum-of-multiples/.meta/Sources/SumOfMultiples/SumOfMultiplesExample.swift

@@ -2,7 +2,7 @@ func toLimit(_ limit: Int, inMultiples: [Int]) -> Int {
   guard limit > 0 else { return 0 }
   var multiples = Set(inMultiples)
 
-  if let indexOfZero = multiples.index(of: 0) {
+  if let indexOfZero = multiples.firstIndex(of: 0) {
     multiples.remove(at: indexOfZero)
   }