Optimize String#== taking character size into account (#15233)

Co-authored-by: Oleh Prypin <[email protected]>

src/string.cr

@@ -3088,8 +3088,18 @@ class String
   # "abcdef".compare("ABCDEF", case_insensitive: true) == 0 # => true
   # ```
   def ==(other : self) : Bool
+    # Quick pointer comparison if both strings are identical references
     return true if same?(other)
-    return false unless bytesize == other.bytesize
+
+    # If the bytesize differs, they cannot be equal
+    return false if bytesize != other.bytesize
+
+    # If the character size of both strings differs, they cannot be equal.
+    # We need to exclude the case that @length of either string might not have
+    # been calculated (indicated by `0`).
+    return false if @length != other.@length && @length != 0 && other.@length != 0
+
+    # All meta data matches up, so we need to compare byte-by-byte.
     to_unsafe.memcmp(other.to_unsafe, bytesize) == 0
   end