record value byte length

Refs #352

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

Author: rvolosatovs

design/mvp/Binary.md

@@ -355,7 +355,7 @@ Notes:
 (See [Value Definitions](Explainer.md#value-definitions) in the explainer.)
 
 ```ebnf
-value                      ::= t:<valtype> v:<val(t)>                  => (value t v)
+value                      ::= t:<valtype> len:<uN> v:<val(t)>         => (value t v) (where len = |v+|)
 val(bool)                  ::= 0x00                                    => false
                              | 0x01                                    => true
 val(u8)                    ::= v:<core:byte>                           => v