Remove excessive header file inclusion from systemDictionary.hpp and others

systemDictionary.hpp is a populer header file (included over 600 times). It includes many files unnecessarily

http://hg.openjdk.java.net/jdk/jdk/file/8ada048df69d/src/hotspot/share/classfile/systemDictionary.hpp

#include "classfile/classLoaderData.hpp"
#include "oops/objArrayOop.hpp"
#include "oops/symbol.hpp"
#include "runtime/java.hpp"
#include "runtime/mutexLocker.hpp"
#include "runtime/reflectionUtils.hpp"
#include "utilities/hashtable.hpp"

It also declares ClassInstanceInfo/ClassLoadInfo which are used by only 10 files. These two classes should be refactored into a new header file.
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signature.hpp is a populer header file (included over 600 times). It uses SymbolTable::new_symbol on a non-critical path. This code should be moved to signature.cpp.

http://hg.openjdk.java.net/jdk/jdk/file/8ada048df69d/src/hotspot/share/runtime/signature.hpp#l134