Support bit fields in structs

JNA should support bit fileds in structs. Currently users are forced to read/set bits by hand, this is error prone and boiler plate.

Given this sample structure:

typedef struct VALUES{
  WORD fVal1  :4;
  WORD fVal2  :2;
  WORD fVal3  :3;
  WORD fDummy  :7;
} 

It would be nice if it could be written like this with JNA:

public static class VALUES extends Structure {
  @BitSize(4) byte fVal1;
  @BitSize(2) byte fVal2;
  @BitSize(3) byte fVal3;
  @BitSize(7) byte fDummy;
  ...
} 

I guess it would not be that simple, cause JNA needs to know the field order and the target type, were the bits should be written to (in this case a 16bit-int).

Start with JNA's Structure field layout and native type handling, then determine how declared bit-sized fields would be ordered and packed into the 16-bit target shown in the C example. Done means the sample VALUES mapping can read and write its stated bit fields without manual masking, with the field order and widths handled consistently.