Disallow U+3000 as the last character of a reserved-body. (#727)

In all places where a 'reserved-body' can occur:
  - in a reserved-statement,
  - after a reserved-annotation or private-use-annotation,
    in a literal-expression, variable-expression, or annotation-expression,
it can be followed by whitespace ('s' nonterminal).

A syntax ambiguity exists, because - as reported in #721 and #725 - a U+3000
character can occur as the last character of a 'reserved-body' (via a
'reserved-char') and also as first character of whitespace ('s' nonterminal).

According to the principles explained in #725, it is not desired that
a 'reserved-body' ends with a U+3000 character; rather, the U+3000 character
is meant to be interpreted as part of the following whitespace.

Test cases (written with \u escapes, for legibility):
For reserved-statement:
.regex   /foo/\u3000\u3000{xyz}{{hello}}
For reserved-annotation:
{ % foo bar \u3000\u3000 }
For private-use-annotation:
{ & foo bar \u3000\u3000 @x }

This patch removes this ambiguity, by disallowing U+3000 as the last character
of a 'reserved-body'.

It thus fixes #725 and the second part of #721.

Details:
  - U+3000 gets removed from 'content-char' and 'reserved-char'.
  - Whereas simple-start-char, text-char, quoted-char stay the same
    (since U+3000 is already part of 's').

Author: bhaible

spec/message.abnf

@@ -95,7 +95,8 @@ content-char      = %x01-08        ; omit NULL (%x00), HTAB (%x09) and LF (%x0A)
                   / %x2F-3F        ; omit @ (%x40)
                   / %x41-5B        ; omit \ (%x5C)
                   / %x5D-7A        ; omit { | } (%x7B-7D)
-                  / %x7E-D7FF      ; omit surrogates
+                  / %x7E-2FFF      ; omit IDEOGRAPHIC SPACE (%x3000)
+                  / %x3001-D7FF    ; omit surrogates
                   / %xE000-10FFFF
 
 ; Character escapes

spec/syntax.md

@@ -311,7 +311,8 @@ content-char      = %x01-08        ; omit NULL (%x00), HTAB (%x09) and LF (%x0A)
                   / %x2F-3F        ; omit @ (%x40)
                   / %x41-5B        ; omit \ (%x5C)
                   / %x5D-7A        ; omit { | } (%x7B-7D)
-                  / %x7E-D7FF      ; omit surrogates
+                  / %x7E-2FFF      ; omit IDEOGRAPHIC SPACE (%x3000)
+                  / %x3001-D7FF    ; omit surrogates
                   / %xE000-10FFFF
 ```