Opening some extensions methods to public

3.1.0 Opening some extensions methods to public (2019-01-13)

Features

• ReadOnlySequence:
  • GetIntLength tests (00327ef4)
  • TryFillSpan method interface and tests (e6c14cef)
  • GetFirst interface and tests. (f73de917)
  • make some methods public (7ef3b6c5)