Implemented method Slice in DataStream

[Megaflan]

Description

Added a Slice method based on the Span<T> seen in C# 8.0 using the new method WriteSegmentTo() as a base.
Using the method Slice will create a new DataStream based on a starting offset parameter or a starting offset parameter + length.

Example

stream.Slice(long start)
stream.Slice(long start, long length)

Closes #143

[pleonex]

Start to look good but we should do the substream instead of copies.

[pleonex]

Code looks perfect. Just one more adjustment to the test to ensure the behavior is what we expect in the design.

[pleonex]

This is impossible to throw excepcion right? Do you want to check if the constructor throw or if the variable is null?

[pleonex]

Can you add just one more test for the base stream. Something like Assert.AreSame with testSlice.BaseStream and stream.BaseStream? so we know there isn't copies

[pleonex]

This PR has been in WIP for 3 years.
Please put a comment or re-open the PR once it's ready for review with the latest comments.
Thanks for the contribution!