Add more methods to Span class

Summary:
This introduces a rudimentary begin(), end(), operator[], and subspan to Span.

This is a partial backport of Core PR13697 : https://github.com/bitcoin/bitcoin/pull/13697/commits/29943a904a11607787d28b1f4288f500bd076dde

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4359