jextract on large Windows header files

[Duncan Gittins]

I've downloaded jdk/panama-foreign and experimented with some changes to 
jextract in order to help with the footprint of jars generated from the 
large Windows header files that I mentioned in the thread: Feedback / 
query on jextract for Windows 10.

1) Added command line args: --symbol {symbol-to-include} and --symbols 
{file-of-symbols-to-include} which eliminates all top level symbols not 
in the parameter list or symbol file(s). This avoids the need to 
generate huge amounts of unnecessary methods.

For example in my application, "jextract shlobj_core.h" gives ~ 11MB jar 
(~ 49k top level symbols), adding -filter for required small group of 
header files cuts this to 880k (5k symbols), and with the --symbols 
filter that is down to a 10K jar (9 symbols) which is much more 
managable size to use. Also the lack of lazy load in --source output is 
also no longer a problem because all static field method handles are 
generally used.

2) Changed "jextract --source" so that identical FunctionDescriptor 
declarations are re-used. This makes the compiled source jar a little 
bit smaller than the generated class jar. For example many of the 
Windows functions such as SetUserObjectInformationA / 
SetUserObjectInformationW have the same descriptor, so jextract can emit 
just one FUNC_ declaration and make MH_  / $FUNC() of the second refer 
to the earlier declaration like this:

     /**  DUPLICATE => SetUserObjectInformationA$FUNC_
     static final FunctionDescriptor SetUserObjectInformationW$FUNC_ = 
FunctionDescriptor.of(C_INT,C_POINTER,C_INT,C_POINTER,C_LONG);
     */
     static final jdk.incubator.foreign.FunctionDescriptor 
SetUserObjectInformationW$FUNC() { return 
SetUserObjectInformationA$FUNC_; }

If you think either of these will help, I can share the small set of 
changes I made.

Duncan