tuple group_by into map

Suppose you have data in the form ` [{"M1", "C1"}, {"M1", "C2"}, {"M2", "C3"}]` and you want a map "grouped by":
`[%{c: ["C2", "C1"], m: "M1"}, %{c: ["C3"], m: "M2"}]`

A helper function (perhaps not optimum :)

```elixir
group_assoc = fn list ->
list
|> Enum.map(fn {m, c} -> %{m: m, c: c} end)
# list_maps = [%{m: "M1", c: "C1"}, %{m: "M1", c: "C2"}, %{m: "M2", c: "C3"}]
|> Enum.group_by(& &1.m)
|> Enum.map(fn {k, v} ->
%{m: k, c: Enum.reduce(v, [], fn e, acc -> [e.c | acc] end)}
end)
end
```

No file or test is named in the issue, so first inspect the repository's existing Elixir utility modules and test structure to find the appropriate entry point. Compare the requested tuple input and grouped map output with existing conventions; done means an agreed helper is implemented with tests covering the shown M1/M2 example.

Written by the indexing model from the issue text.