[io grant] Straighten out &slurp/&spurt/&get/&getc/&close
[io grant] Straighten out &slurp/&spurt/&get/&getc/&close

- Pass all args via captures, not just some
- Fail, not throw
- Add IO::Handle candidate for spurt, to maintain consistency
- Change Cool:D slurp candidate to IO()
- Add type constraints on subs that are missing it
    (IO::ArgFiles is IO::Handle)

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