[foreign] RFC: Additional jextract filtering

[Jorn Vernee]

Hi,

I've been looking into adding more filtering capabilities to jextract 
over the past few days.

I have come up with the following proposal; Replace --include-symbols 
and --exclude-symbols with 2 new options, --include and --exclude. These 
options take as an argument a filter expression, which looks like 
SELECTOR:PATTERN or, for includes only, SELECTOR-PRESET, where SELECTOR 
is one of [symbol, macro, struct, typedef, enum], and PRESET is one of 
[ALL, REQUIRED]. PATTERN is a regex pattern. The selector lets us select 
the type of element a pattern applies to, and the preset is a default 
for what to do when there are no explicit includes specified for that 
specific selector.

The earlier mentioned problem with allowing filtering of things besides 
symbols and macros (which is what the current implementation supports), 
is that we might end up excluding something that is actually required. 
So, to make sure that doesn't happen, we first filter a set of 'root' 
declarations, which are library symbols (global vars and functions) and 
macros, then from those compute a minimal set of dependencies, and then 
use that in a subsequent round of filtering for structs, typedefs and 
enums to make sure nothing that is required gets filtered out. This 
minimal dependency set can also be used to only include what's 
absolutely needed, by using the REQUIRED filter preset.

To give an example; If we have a header like so:

```
struct Foo {
     int x;
};

struct Bar {
     int x;
};

void func(struct Foo foo);
```

We can use the option `--include struct-REQUIRED`, and `Bar` will be 
omitted from the output, since it is not used directly by `func`. 
However, if we also use the option `--exclude struct:Foo` the output 
won't change, since the struct Foo is required by the function func. 
But, if we instead use `--exclude symbol:func` in the options, we get no 
output at all, because `func` is filtered, and then none of the structs 
are required, so they get filtered as well.

I have implemented a prototype of this proposal here: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/filters/webrev.00/

If the proposal sounds good I can add more tests and make an RFR.

Thanks,
Jorn