see #371
This commit does not implement all features discussed in this issue.
It has implemented it as a macro expansion. Thus, the following is
accepted
```lean
inductive StrOrNum where
| S (s : String)
| I (i : Int)
def StrOrNum.asString (x : StrOrNum) :=
match x with
| I a | S a => toString a
```
It may confuse the Lean LSP server. The `a` on `toString` shows the
information for the first alternative after expansion (i.e., `a` is an `Int`).
After expansion, we have
```
def StrOrNum.asString (x : StrOrNum) :=
match x with
| I a => toString a
| S a => toString a
```
