Add type restrictions to io

[Vici37]

This is the output of compiling cr-source-typer and running it with the below incantation:

CRYSTAL_PATH="./src" ./typer spec/std_spec.cr \
  --error-trace --exclude src/crystal/ \
  --stats --progress \
  --union-size-threshold 2 \
  --ignore-private-defs \
  --ignore-protected-defs \
  src/io

This is related to https://github.com/crystal-lang/crystal/pull/15682 .

I also reran the above command but without the --union-size-threshold 2, and updated several long union types to IO by itself.