Duralite: cache directory listing

[tkellogg]

Reduce disk I/O by moving most directory listing to a probabalistic
cache approach. For one, I don't want to wear out disks by constantly
accessing them. Spinning disks are especially problematic because
latency can be quite high.

This introduces CachedDirIter which replaces usages of fs::ReadDir.
It's an enum that can either wrap a fs::ReadDir or represent a cache
hit by iterating a vector af paths. The cache itself is a Trie, so it
shouldn't take that much memory to hold in memory all paths under
$HOME, for example. The cache lives at program scope and is passed
down into where it's needed.

Cache invalidation is a problem, though. I don't want to refresh the
entire Trie all at once, but I also want guarantees that a new directory
will be recognized within a certain time limit, e.g. within 10 minutes.
Here I take a probabalistic approach. Each individual directory entry is
invalidated independently. On any given pass, there's an X% chance
that a single directory will be invalidated. X is calculated such that
caches will be invalidated within some maximum time bound (10 minutes)
95% of the time. In the remaining 10% of cases, they're
force-invalidated at the 10 minute mark.

The duralite sub-project has been about reducing dura's presence on host
machines by utilizing as few resources as possible. I don't want people
to not use dura because "my computer runs slow with it". Something I've
been observing is that, as I reduce these I/O-intensive bottlenecks,
dura uses even more CPU. In a follow-up PR I want to add some
strategically placed thread::sleep's to spread out the CPU usage
evenly throughout the entire 5 second polling interval.

[tkellogg]

TODO: this needs more tests before it can be released into the wild