use capitalized variable only as data constructor

[m-shaka]

Like Haskell and Elm. For example, statements below should raise an error.

Hoge  = "hoge"

In Haskell, the error is semantic one, i.e. "not in scope" error, while in Elm is syntactic one (parse error).

[kirisaki]

I was thinking about what deal it.
The parser doesn't pass not capitalized data constructor,
so I suppose this problem has low priority.

[kirisaki]

I'm rewriting the parser, and I want to separate them...
Though, I should pass the tests first.