add FileKeyIndex to Keys db to optimize getAssociatedKey

This is a schema change so will break any existing keys databases. But,
it's not been released yet, so I'm still able to make such changes.

This speeds up the benchmark quite nicely:

benchmarking keys database/getAssociatedKey from 1000 (hit)
time                 91.65 μs   (91.48 μs .. 91.81 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 91.78 μs   (91.66 μs .. 91.94 μs)
std dev              468.3 ns   (353.1 ns .. 624.3 ns)

benchmarking keys database/getAssociatedKey from 1000 (miss)
time                 53.33 μs   (53.23 μs .. 53.40 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 53.43 μs   (53.36 μs .. 53.53 μs)
std dev              274.2 ns   (211.7 ns .. 361.5 ns)

benchmarking keys database/getAssociatedKey from 10000 (hit)
time                 92.99 μs   (92.74 μs .. 93.27 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 92.90 μs   (92.76 μs .. 93.16 μs)
std dev              608.7 ns   (404.1 ns .. 963.5 ns)

benchmarking keys database/getAssociatedKey from 10000 (miss)
time                 53.12 μs   (52.91 μs .. 53.39 μs)
                     1.000 R²   (0.999 R² .. 1.000 R²)
mean                 52.84 μs   (52.68 μs .. 53.16 μs)
std dev              715.4 ns   (400.4 ns .. 1.370 μs)

Database/Keys/SQL.hs

@@ -28,6 +28,7 @@ Associated
   key SKey
   file FilePath
   KeyFileIndex key file
+  FileKeyIndex file key
 Content
   key SKey
   cache SInodeCache