Implement SequenceReader.TryPeek(long offset, out T value)

[MarcoRossignoli]

Closes https://github.com/dotnet/corefx/issues/40845

cc: @JeremyKuhne

[JeremyKuhne]

Thanks for tackling this! I'd optimize for peeking within the current span, then fall back to using the internal TryCopyMultisegment as discussed inline.

The key design point is to make any in span operations as fast as possible. When we cross spans we should still try to be efficient, but it is okay to not be micro-optimized and favor code-reuse / clarity.

[maryamariyan]

Thank you for your contribution. As announced in dotnet/coreclr#27549 this repository will be moving to dotnet/runtime on November 13. If you would like to continue working on this PR after this date, the easiest way to move the change to dotnet/runtime is:

1. In your corefx repository clone, create patch by running git format-patch origin
2. In your runtime repository clone, apply the patch by running git apply --directory src/corefx <path to the patch created in step 1>

[JeremyKuhne]

Sorry about the unnecessary confusion. Looks good with just some minor issues with comments (notably capitalizing the first word). You should also be validating that we get default out when the method returns false.

[MarcoRossignoli]

@JeremyKuhne ready to review, I'll take some other api after that.

[JeremyKuhne]

Thanks!

[davidfowl]

Awesome!

[ahsonkhan]

Some late review nits.

[MarcoRossignoli]

thank's @ahsonkhan for review...bit busy days I'll take a look starting in middle of next week if it's not a problem, it's on my backlog now.