Don't allow escaped surrogated identifiers + introduce ascii_identifiers

Don't use 2 characters for surrogates in identifiers because there is
support for the \u{} syntax when escaped identifiers were introduced.

Also catch eof errors while reading identifier names

Introduce ascii_identifiers:

By setting ascii_identifiers to undefined (default value),
ascii_identifiers will print identifiers using the same setting as
ascii_only within the limits of the ecmascript 6 grammar.

ascii_identifiers accept true and false, allowing identifiers to be
printed under different settings than strings with the ascii_only setting.

lib/parse.js

@@ -551,7 +551,7 @@ function tokenizer($TEXT, filename, html5_comments, shebang) {
         return next_token;
     });
 
-    function read_name() {
+    var read_name = with_eof_error("SyntaxError: Unterminated identifier name", function() {
         var name = "", ch, escaped = false, hex;
         var read_escaped_identifier_char = function() {
             escaped = true;
@@ -593,7 +593,7 @@ function tokenizer($TEXT, filename, html5_comments, shebang) {
             parse_error("SyntaxError: Escaped characters are not allowed in keywords");
         }
         return name;
-    };
+    });
 
     var read_regexp = with_eof_error("SyntaxError: Unterminated regular expression", function(regexp){
         var prev_backslash = false, ch, in_class = false;