Dedup -I flags.

The current code leads to unpleasant makefiles which leads to unpleasant build logs. Also made the code more declarative... that use of `Hashtbl` was creative. 

```
bin-insights.comp-byte            ?= $(comp-byte) \
                                     $(shell ocamlfind query -r -predicates byte -format "-I %d" -r uutf useri.tsdl useri react otfm lit.u lit.gl lit gg fut.u fut.select fut.r fut dicomm | uniq) \
                                     -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I 
$(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I
 $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I
 $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I
 $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I
 $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I
 $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I
 $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I
 $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I $(BUILDIR)/bin-insights -I
 $(BUILDIR)/bin-insights
```