Support pattern using bound variables

[yoshihiro503]

In Erlang, the pattern using bound variables work as like constant pattern.
For example:

X = 1,
case 123 of
  X -> X + 1
end.

↓

** exception error: no case clause matching 123

For this, we must consider the second binding of a already bound variable.

The case of shadowing variables must also be considered, but it is a different PR #135.