Fix bugs in incremental facet indexing with variable parameters

e.g. add one facet value incrementally with a group_size = X and then
add another one with group_size = Y

It is not actually possible to do so with the public API of milli,
but I wanted to make sure the algorithm worked well in those cases
anyway.

The bugs were found by fuzzing the code with fuzzcheck, which I've added
to milli as a conditional dev-dependency. But it can be removed later.

milli/Cargo.toml

@@ -56,6 +56,9 @@ maplit = "1.0.2"
 md5 = "0.7.0"
 rand = {version = "0.8.5", features = ["small_rng"] }
 
+[target.'cfg(fuzzing)'.dev-dependencies]
+fuzzcheck = { path = "../../fuzzcheck-rs/fuzzcheck" }
+
 [features]
 default = [ "charabia/default" ]