Implement support for BITPOS (bitmap command)

[cunla]

#93

[github-actions]

Coverage report:

Name                                              Stmts   Miss Branch BrPart  Cover
-----------------------------------------------------------------------------------
fakeredis/__init__.py                                 6      2      0      0    67%
fakeredis/_basefakesocket.py                        225      9     96     10    94%
fakeredis/_commands.py                              295      8    110      5    96%
fakeredis/_fakesocket.py                             17      0      0      0   100%
fakeredis/_helpers.py                               168      4     61      3    97%
fakeredis/_msgs.py                                   60      0      0      0   100%
fakeredis/_server.py                                122      6     42      5    93%
fakeredis/_zset.py                                   64      0     16      0   100%
fakeredis/aioredis.py                               155      8     43      7    92%
fakeredis/commands_mixins/__init__.py                 0      0      0      0   100%
fakeredis/commands_mixins/bitmap_mixin.py           115      0     56      0   100%
fakeredis/commands_mixins/connection_mixin.py        21      0      4      0   100%
fakeredis/commands_mixins/generic_mixin.py          244     17    110     10    91%
fakeredis/commands_mixins/hash_mixin.py              82      1     16      1    98%
fakeredis/commands_mixins/list_mixin.py             176      5     76      5    96%
fakeredis/commands_mixins/pubsub_mixin.py            81      0     36      6    95%
fakeredis/commands_mixins/scripting_mixin.py        147      3     70      4    97%
fakeredis/commands_mixins/server_mixin.py            49      0     14      0   100%
fakeredis/commands_mixins/set_mixin.py              139     12     58      6    88%
fakeredis/commands_mixins/sortedset_mixin.py        326      5    137      4    98%
fakeredis/commands_mixins/string_mixin.py           254      5    126      6    97%
fakeredis/commands_mixins/transactions_mixin.py      71      2     20      1    97%
fakeredis/stack/__init__.py                           7      3      0      0    57%
fakeredis/stack/_json_mixin.py                      189      7     85      3    96%
-----------------------------------------------------------------------------------
TOTAL                                              3013     97   1176     76    96%