Fix "done" method of generators.

[ghuls]

Fix "done" method of generators as before it would segfault in debug mode when getting the next element when the iterator was finished.

$ cat test_iter.codon
b = [1, 2, 3]

d = iter(b)
print(min(d))
print(d.done())
print(min(d))

$ /software/exaloop/codon/codon-0.19.4/bin/codon run test_iter.codon
1
Segmentation fault (core dumped)

$ /software/exaloop/codon/codon-0.19.4_done_fix/bin/codon run test_iter.codon
1
True
ValueError: min() arg is an empty sequence

Raised from: std.internal.builtin.min.0:0
/software/exaloop/codon/codon-0.19.4_pipe/lib/codon/stdlib/internal/builtin.codon:83:13

Backtrace:
  [0x7f9950a9986f] std.internal.builtin.min.0:0[Tuple[Generator[int]],Optional[NoneType],Optional[NoneType]].851 at /software/exaloop/codon/codon-0.19.4_pipe/lib/codon/stdlib/internal/builtin.codon:83:13
  [0x7f9950a9e8d2] main.0 at /software/exaloop/codon/test_iter.codon:6:7
Aborted (core dumped)

Fixes: #246