scratch space: use single allocation

Summary:

• scratch: add magic bytes to beginning of structure
• scratch space: thread error_callback into all scratch space functions

Use it when checking magic bytes

• scratch: unify allocations
• scratch: rename max_size to size, document that extra will actually be allocated
• scratch: save a couple bytes of unnecessarily-allocated memory
• scratch: replace frames with "checkpoint" system

This is a backport of secp256k1 PR600

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5146