perf: optimize MiniFAT free sector management

[francisdb]

• same as feat: improve writing speed #72 but for the MiniFAT
• Adds a worst case benchmark for MiniFAT write

benchmark result:

write MiniFAT max streams/total/default
                        time:   [51.553 ms 51.682 ms 51.865 ms]
                        thrpt:  [37.649 MiB/s 37.782 MiB/s 37.876 MiB/s]
                 change:
                        time:   [−67.689% −67.435% −67.222%] (p = 0.00 < 0.05)
                        thrpt:  [+205.08% +207.07% +209.49%]
                        Performance has improved.

[mdsteele]

Thanks!