Merge duplicate constant declarations into a union type

[pvcresin]

When a constant is assigned more than once, Service#dump_declarations
emitted one declaration line per assignment, so

class C
  D = 1
  D = "str"
end

was dumped as two declarations for the same constant:

class C
  D: Integer
  D: String
end

This deduplicates constant writes by their static cpath so that each
constant yields a single declaration, and shows the union of its types
when it is assigned more than once:

class C
  D: (Integer | String)
end

A single assignment keeps using the assigned value (node.ret) rather
than the constant entity's type, so that a same-named class or module
(e.g. one declared in RBS) is not mixed into the declaration; see
scenario/rbs/remove-class.rb, where C = 1 coexists with an RBS
class C and the declaration is expected to stay C: Integer. The
deduplication mirrors the existing seen_ivars/seen_cvars/seen_gvars
handling in the same method.

scenario/known-issues/multi-const-write.rb now passes and is promoted
to scenario/const/. scenario/variable/operator_write.rb is updated
since it previously encoded the duplicated output (D = 0; D += 1 and
C::E = 0; C::E += 1 now each produce a single line).

All 429 tests pass (bundle exec rake test).

[sinsoku]

It looks good!