Add --tallies option to crystal tool unreachable

[straight-shoota]

This option changes crystal tool unreachable to output all defs and their call counts. The call count of unreachable defs is zero. This additional information enables collecting data across multiple entrypoints with different dependency graphs.

Consider two files serving as different entry points, entry-a.cr and entry-b.cr. They require some of the same file dependencies, but also different ones. The standard behaviour of unreachable can only tell which defs are unreachable in a and which are unreachable in b. Shared entries are definitely unreachable in both. A def ::foo that is only listed as unreachable in a, not in b can mean it's reachable in b. But it can also mean ::foo is not even in the dependency graph of entry-b.cr and thus never seen by the compiler.
The --tallies option printing all defs, reachable and unreachable, resolves this uncertainty by clearly stating whether ::foo is present in b or not.
For this use case, the exact call count wouldn't be necessary. The information whether there's at least one call would suffice. But giving a total number isn't much extra effort and can be useful for further statistical analysis.

The implementation overall is pretty simple, the main change is from Set to register presence to a Hash for counting occurences.

A demonstration of applying this features on a code base is available at https://gist.github.com/straight-shoota/f7c9120b3bdbc067b4877f88def9c51e
It analyzes the source code of the compiler and standard library using three entry points (compiler, compiler_spec and std_spec) and combines the information into a single database.