special-case s8 and u8 in Binary.md

Special case `s8` and `u8` to ensure that exactly 1 byte is used
to represent values of those types

#336 (comment)

Signed-off-by: Roman Volosatovs <rvolosatovs@riseup.net>

Author: rvolosatovs

design/mvp/Binary.md

@@ -358,8 +358,8 @@ Notes:
 value                      ::= t:<valtype> v:<val(t)>                  => (value t v)
 val(bool)                  ::= 0x00                                    => false
                              | 0x01                                    => true
-val(u8)                    ::= v:<core:u8>                             => v
-val(s8)                    ::= v:<core:s8>                             => v
+val(u8)                    ::= v:<core:byte>                           => v
+val(s8)                    ::= v:<core:byte>                           => v if v < 128 else (v - 256)
 val(s16)                   ::= v:<core:s16>                            => v
 val(u16)                   ::= v:<core:u16>                            => v
 val(s32)                   ::= v:<core:s32>                            => v