Support symbol-named properties in API

Support symbol-named properties in API

Add new "Name" type to API that is a supertype of Symbol and String.

Object::SetDeclaredAccessor, Object::SetAccessorProperty, Template::Set,
Template::SetAccessorProperty, and Template::SetDeclaredAccessor now
take a Name as the property name instead of a String.

Add Object::SetAccessor, Template::SetNativeDataProperty, and
ObjectTemplate::SetAccessor overloads that can define accessors for
symbol-named properties.

ObjectTemplate::SetGenericNamedPropertyHandler is a new version of
ObjectTemplate::SetNamedPropertyHandler that can intercept any Name
property access, not just access to String properties.  Eventually we
will deprecate and remove ObjectTemplate::SetNamedPropertyHandler, then
rename ObjectTemplate::SetGenericNamedPropertyHandler back to
ObjectTemplate::SetNamedPropertyHandler.

R=rossberg@chromium.org
BUG=v8:3394
TEST=cctest/test-api/TestSymbolProperties
LOG=Y

[wingo, 2014-08-12 12:13:45]

The accessor part of this is superceded by
https://codereview.chromium.org/459413002/.  I'll post the interceptor part
later, if the accessor part goes through.