[WIP] io: update Reader interface

[kt3k]

This PR updates the Reader interface based on the change introduced by denoland/deno#2591

[piscisaureus]

Looking great.
@ry since this needs to land in concert with changes to deno (we can't get either to pass CI independently), how should we go about landing this?

I would suggest to make a simple patch that just adds the EOF symbol to deno and nothing else.
Then release that.
Then land this.
Then remove ReadResult from Deno.

[kt3k]

@piscisaureus

I would suggest to make a simple patch that just adds the EOF symbol to deno and nothing else.
Then release that.
Then land this.

This PR depends on the change of Reader interface. Only adding EOF symbol to deno core is not enough for passing the CI of this branch.

I would suggest the following:

1. land io: change Reader interface deno#2591
2. release the next version of deno core
3. update deno in this branch (CI should pass here)
4. land this branch

[ry]

I've updated to v0.11.0 but the CI is still failing on Windows at emptyDirIfItExist

AssertionError: Expected function to throw.
    at assertThrowsAsync (file:///D:/a/1/s/testing/asserts.ts:221:15)
    at async emptyDirIfItExist (file:///D:/a/1/s/fs/empty_dir_test.ts:57:9)
    at async runTestsSerial (file:///D:/a/1/s/testing/mod.ts:175:13)
    at async runTests (file:///D:/a/1/s/testing/mod.ts:223:9)
    at async run (file:///D:/a/1/s/test.ts:32:5)

[ry]

LGTM - landing this despite the broken test so that we can quickly fix std for v0.11.0

Thanks @kt3k !