bitmap: Allow bitmap type to be forward declared

bitmap: Allow bitmap type to be forward declared

Currently bitmap type is defined via untagged struct which makes it
impossible to forward declare it. Forward-declaring is useful since all
bitmap functions only use bitmap* and in public user-visible
headers/datastructures it is enough to indicate that a data field with
bitmap pointer is there, whereas bitmap.h can be included only in
implementation.

Beside that some headers are included by both C and C++ parts of a
project, and when ccan/bitmap.h is processed by C++ compiler it gives:

    ./3rdparty/ccan/ccan/bitmap/bitmap.h: In function ‘bitmap* bitmap_alloc(long unsigned int)’:
    ./3rdparty/ccan/ccan/bitmap/bitmap.h:201:15: error: invalid conversion from ‘void*’ to ‘bitmap*’ [-fpermissive]
      return malloc(bitmap_sizeof(nbits));
             ~~~~~~^~~~~~~~~~~~~~~~~~~~~~
    ./3rdparty/ccan/ccan/bitmap/bitmap.h: In function ‘bitmap* bitmap_realloc0(bitmap*, long unsigned int, long unsigned int)’:
    ./3rdparty/ccan/ccan/bitmap/bitmap.h:227:18: error: invalid conversion from ‘void*’ to ‘bitmap*’ [-fpermissive]
      bitmap = realloc(bitmap, bitmap_sizeof(nbits));
               ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    ./3rdparty/ccan/ccan/bitmap/bitmap.h: In function ‘bitmap* bitmap_realloc1(bitmap*, long unsigned int, long unsigned int)’:
    ./3rdparty/ccan/ccan/bitmap/bitmap.h:238:18: error: invalid conversion from ‘void*’ to ‘bitmap*’ [-fpermissive]
      bitmap = realloc(bitmap, bitmap_sizeof(nbits));

-> Give to users ability not to force-include ccan/bitmap.h by
forward-declaring bitmaps like this:

    typedef struct bitmap bitmap;
    ...
    struct MyStruct {
        bitmap *my_bitmap;
    };

Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
Message-Id: <20191021150903.25159-1-kirr@nexedi.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>