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/src/lib.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(all(test, fuzzing), feature(no_coverage))]
+
 #[macro_use]
 pub mod documents;