Cache the mount table for -fstype

[tavianator]

#408 implemented -fstype, but it's very slow since it re-reads the mount table every time:

Benchmark 1: find ~/code/linux -fstype btrfs
  Time (mean ± σ):      97.9 ms ±   3.2 ms    [User: 31.0 ms, System: 66.5 ms]
  Range (min … max):    91.4 ms … 104.6 ms    30 runs
 
Benchmark 2: ./target/release/find ~/code/linux -fstype btrfs
  Time (mean ± σ):      6.939 s ±  0.840 s    [User: 2.506 s, System: 3.826 s]
  Range (min … max):    6.098 s …  8.947 s    10 runs
 
Summary
  find ~/code/linux -fstype btrfs ran
   70.89 ± 8.89 times faster than ./target/release/find ~/code/linux -fstype btrfs