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/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;
     }