Fix some memory leaks

[phischu]

@marvinborner With these fixes the following does not leak anymore:

import array

def main() = {
  array::allocate[Int](1)
  ()
}

Does this also fix other tests that are commented out?

[marvinborner]

No, the other leaking tests are not fixed by this entirely. My additional pos erasers in the stdlib make Valgrind even angrier.

[phischu]

Could you please add your additional pos erasers in the stdlib to this pull request?

[marvinborner]

I've added them. I'm still not 100% sure when we are supposed to add erasers and whether my additions are correct. We should really start writing a detailed sharer/eraser guide :)

[phischu]

Thank you. I have applied some fixes, please check if we can enable more tests for llvm now.

[marvinborner]

This seems to improve things by a lot! There are still some tests left that fail with segfaults (use-after-free most probably) in UV/IO code, as well as some IO/bytearray-related code in stdlib. I've pushed the uncommented tests to see some immediate CI response on further fixes.

[jiribenes]

Does this fix the leak in #711?

[phischu]

examples/stdlib/io/filesystem/files.effekt (llvm) and examples/stdlib/io/filesystem/async_file_io.effekt (llvm) fail with "Syscall param write(buf) points to uninitialised byte(s)", @marvinborner could you investigate? I'll try to hunt down the other ones.