Update IO instance methods and constants

Author: sampersand

core/builtin.rbs

@@ -182,6 +182,7 @@ interface _Reader
   def read: (?int? length, ?string outbuf) -> String?
 end
 
+%a{steep:deprecated} # now lives in `IO`
 interface _ReaderPartial
   def readpartial: (int maxlen, ?string outbuf) -> String
 end
@@ -192,6 +193,7 @@ interface _Writer
   def write: (*_ToS data) -> Integer
 end
 
+%a{steep:deprecated} # not used at all
 interface _Rewindable
   # Positions the stream to the beginning of input, resetting `lineno` to zero.
   #

core/encoding.rbs

@@ -28,6 +28,13 @@
 # useful when you use other ASCII-compatible encodings.
 #
 class Encoding
+  # The `_EncodeFallbackAref` is used in functions such as `String::encode` to get the fallback
+  # value when a charcater can't be converted between encodings.
+  interface _EncodeFallbackAref
+    # Returns a string to use as a replacement, or `nil` if a replacement isn't possible.
+    def []: (String) -> string?
+  end
+
   def self._load: [T] (T) -> T
 
   # <!--