passwd: Cache the user/group tables

This is a significant optimization for conditions that need these
tables:

    Benchmark #1: ./bfs ~/code/linux -nouser >/dev/null
      Time (mean ± σ):     232.0 ms ±   2.5 ms    [User: 44.3 ms, System: 185.0 ms]
      Range (min … max):   228.7 ms … 238.7 ms    12 runs

    Benchmark #2: ./bfs-1.6 ~/code/linux -nouser >/dev/null
      Time (mean ± σ):      1.050 s ±  0.012 s    [User: 544.2 ms, System: 500.0 ms]
      Range (min … max):    1.025 s …  1.063 s    10 runs

    Benchmark #3: find ~/code/linux -nouser >/dev/null
      Time (mean ± σ):      1.040 s ±  0.012 s    [User: 533.6 ms, System: 500.6 ms]
      Range (min … max):    1.017 s …  1.054 s    10 runs

    Summary
      './bfs ~/code/linux -nouser >/dev/null' ran
        4.48 ± 0.07 times faster than 'find ~/code/linux -nouser >/dev/null'
        4.52 ± 0.07 times faster than './bfs-1.6 ~/code/linux -nouser >/dev/null'

1 files changed, 1 insertions, 0 deletions

diff --git a/main.c b/main.c
index b8d7bae..442f66a 100644
--- a/main.c
+++ b/main.c
@@ -40,6 +40,7 @@
  *     - dstring.[ch]  (a dynamic string library)
  *     - fsade.[ch]    (a facade over non-standard filesystem features)
  *     - mtab.[ch]     (parses the system's mount table)
+ *     - passwd.[ch]   (a cache for the user/group tables)
  *     - spawn.[ch]    (spawns processes)
  *     - stat.[ch]     (wraps stat(), or statx() on Linux)
  *     - time.[ch]     (date/time handling utilities)