Consider undeclared use of `_` dead variable

[rebcabin]

Perhaps the following is preferable:

    for _ in range(10):
        t = get_non_white(lexer)
        assert t == EOF_SIGNAL

to this, which is currently required:

    _ : i32
    for _ in range(10):
        t = get_non_white(lexer)
        assert t == EOF_SIGNAL

[certik]

Furthermore, the _ can be redefined, so each _ can internally be represented by a local variable with a unique name and correct inferred type.