if statement suppresses error for ignoring return value

[thejoshwolfe]

fn foo() -> %void {}
fn main() {
    foo(); // error: return value ignored
    if (true) foo(); // no complaint
}

This mistake is made in elf.zig:236

I have a suspicion this will be solved by the code changes required by #287.

[thejoshwolfe]

to follow up the issue in elf.zig mentioned above, it appears that io.InStream.close not longer returns an error, so that's fine.

but i did fix a bunch of other cases where this problem was happening, mostly by assigning to _.