Add IO#read_greedy

[BlobCodes]

Closes #14605

Adds the proposed #read_greedy method which tries to fill a Slice multiple times if it was only filled partially by the read method.

Since some users seem to expect this kind of behaviour from #read (leading to hard-to-debug bugs), this could be seen as a "safer" version of #read for some use-cases.

I didn't try to add a lazy reading function as there would be too many interpretations of what reading lazily even means depending on the use-case.

There's also a small optimization for IO::Memory (not benchmarked).

[straight-shoota]

We should add some specs for this new method.

[crysbot]

This pull request has been mentioned on Crystal Forum. There might be relevant details there:

https://forum.crystal-lang.org/t/empty-content-when-reading-big-packets-on-tcp-ip/8652/4

[straight-shoota]

suggestion: We could refactor the implementation of #read_fully? to use #read_greedy:

  def read_fully?(slice : Bytes) : Int32?
    count = read_greedy(slice)
    return nil unless count == slice.size
    count
  end

This could also be a follow-up.

[BlobCodes]

I applied all suggestions and added the cross-reference to the docs.