lib: add copy function for bitfield_t

lib: add copy function for bitfield_t

Add a function to copy a bitfield_t structure.

Add a ‘void *’ to ‘word_t *’ converstion in bf_init() to avoid the
following error:

> ./lib/bitfield.h: In function ‘bf_copy’:
> ./lib/bitfield.h:75:12: error: request for implicit conversion from ‘void *’ to ‘word_t *’ {aka ‘unsigned int *’} not permitted in C++ [-Werror=c++-compat]
>    (v).data = XCALLOC(MTYPE_BITFIELD, ((v).m * sizeof(word_t)));  \
>             ^
> ./lib/bitfield.h:278:2: note: in expansion of macro ‘bf_init’
>   bf_init(dst, WORD_SIZE * (src.m - 1));
>   ^~~~~~~

Signed-off-by: Hiroki Shirokura <hiroki.shirokura@linecorp.com>
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>