DeJson string deserialization accepts various invalid JSON strings

[Thom Chiovoloni]

After #10 lands, there are two issues with DeJson's string deserialization code:

1. Various invalid escapes to get through. Any character that appears after a backslash is allowed more or less.
2. Various ASCII control codes are allowed in JSON strings in unescaped form, but it should be an error if any below U+0020 appear without being escaped. (E.g.
   

   nanoserde/src/serde_json.rs

   Lines 517 to 519 in 0704316

    '\0' => { 
        return Err(self.err_parse("string")); 
    } 

   and

   nanoserde/src/serde_json.rs

   Lines 524 to 526 in 0704316

    if self.cur == '\0' { 
        return Err(self.err_parse("string")); 
    } 

   should be adjusted so that it's any character between ('\u{0}'..'\u{20}') that causes an error).

I don't really care a ton about either of these personally, though.