Introduce Span type and use it instead of FLATDATA

Summary:

• Add Slice: a (pointer, size) array view that acts like a container
• Support serializing Span<unsigned char> and use that instead of FLATDATA

This is a backport of Core PR12886

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4199