fix compatibility with new compiler constrainst

src/message_pack/lexer.cr

@@ -14,7 +14,7 @@ class MessagePack::Lexer
     @io = io
     @token = Token.new
     @byte_number = 0
-    @current_byte = 0
+    @current_byte = 0_u8
     @eof = false
   end
 

src/message_pack/token.cr

@@ -12,6 +12,9 @@ class MessagePack::Token
   property :used
 
   @size : Int64
+  @int_value : Int::Signed
+  @uint_value : Int::Unsigned
+  @float_value : Float32 | Float64
 
   def initialize
     @type = :EOF

src/message_pack/unpack_exception.cr

@@ -1,7 +1,7 @@
 # Raised on invalid MessagePack strings.
 class MessagePack::UnpackException < Exception
   # The line number where the invalid MessagePack was detected.
-  getter byte_number
+  getter byte_number : Int32
 
   # Creates a ParseException with the given message and byte number.
   def initialize(message, @byte_number)