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 ParseFunctionLiteral(),
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 ParseExpression() 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.

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.