Use ReadOnlySpan<char>.Split() (#557)

- Treat some strings as spans and use their `Split()` method.
- Use `string.Join(char)`.
- Rename variable to theta symbol.

src/ProjectEuler/Puzzles/Puzzle039.cs

@@ -32,9 +32,9 @@ internal static ICollection<string> Solve(int perimeter)
             for (double b = 1; b < perimeter - a - 1; b++)
             {
                 double tangent = a / b;
-                double theta = Math.Atan(tangent);
+                double θ = Math.Atan(tangent);
 
-                double c = a / Math.Sin(theta);
+                double c = a / Math.Sin(θ);
 
                 if (Math.Abs(a + b + c - perimeterDouble) < double.Epsilon)
                 {
@@ -45,7 +45,7 @@ internal static ICollection<string> Solve(int perimeter)
                         c.ToString(Format, formatProvider),
                     ];
 
-                    string solution = $"{{{string.Join(",", sides.Order())}}}";
+                    string solution = '{' + string.Join(',', sides.Order()) + '}';
 
                     solutions.Add(solution);
                 }

src/ProjectEuler/Puzzles/Puzzle042.cs

@@ -51,17 +51,16 @@ internal IList<string> ReadWords()
         using var stream = ReadResource();
         using var reader = new StreamReader(stream);
 
-        string rawWords = reader.ReadToEnd();
+        var text = reader.ReadToEnd().AsSpan();
+        var words = new List<string>();
 
-        string[] split = rawWords.Split(',');
-
-        var words = new List<string>(split.Length);
-
-        foreach (string word in split)
+        foreach (var word in text.Split(','))
         {
-            words.Add(word.Trim('\"'));
+            words.Add(text[word].Trim('\"').ToString());
         }
 
+        words.TrimExcess();
+
         return words;
     }