`unused_io_amount` captures `Ok(_)`s

Partial rewrite of `unused_io_account` to lint over Ok(_).

Moved the check to `check_block` to simplify context checking for
expressions and allow us to check only some expressions.

For match (expr, arms) we emit a lint for io ops used on `expr` when an
arm is `Ok(_)`. Also considers the cases when there are guards in the
arms. It also captures `if let Ok(_) = ...` cases.

For `Ok(_)` it emits a note indicating where the value is ignored.

changelog: False Negatives [`unused_io_amount`]: Extended
`unused_io_amount` to catch `Ok(_)`s in `If let` and match exprs.

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