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.

.gitignore

@@ -2,6 +2,8 @@
 /target
 /Cargo.lock
 
+milli/target/
+
 # datasets
 *.csv
 *.mmdb