Implement handling of arrow functions in the parser

Arrow functions are parsed from ParseAssignmentExpression(). Handling the
parameter list is done by letting ParseConditionalExpression() parse a comma
separated list of identifiers, and it returns a tree of BinaryOperation nodes
with VariableProxy leaves, or a single VariableProxy if there is only one
parameter. When the arrow token "=>" is found, the VariableProxy nodes are
passed to ParseArrowFunctionLiteral(), which will then skip parsing the
paramaeter list. This avoids having to rewind when the arrow is found and
restart parsing the parameter list. Note that ParseAssignmentExpression()
expects parenthesized expressions to not be empty, so checking for a closing
parenthesis is added in handling the empty parameter list "()" will accept a
right-paren and return an empty expression, which means that the parameter
list is empty.

The implementation is done in ParserBase, so it was needed to do some
additions to ParserBase, ParserTraits and PreParserTraits. Some of the
glue code can be removed later on when more more functionality is moved
to ParserBase.

Additionally, this adds the following machinery:

- A runtime flag "harmony_arrow_functions" (disabled by default).
  Enabling "harmony" will enable it as well.
- An IsArrow bit in SharedFunctionInfo, and accessors for it.
- An IsArrow bit in FunctionLiteral, accessorts for it, and
  a constructor parameter to set its value.
- In ParserBase: allow_arrow_functions() and set_allow_arrow_functions()
- A V8 native %FunctionIsArrow(), which is used to skip adding the
  "function " prefix when getting the source code for an arrow
  function.