sys: use typedef for `struct bloom_t`

`bloom_t` is defined as a struct.
`_t` can mislead the user to think of bloom_t
as a typedef (see our coding conventions) instead of a struct.
Thus, I modified `struct bloom_t` to be a *typedefed* struct.

Another solution would be to rename bloom_t to sth. like bloom_s
everywhere and use `struct bloom_s` instead of `bloom_t`.