Enable context creation in preallocated memory

Summary:

• Prepare for manual memory management in preallocated memory
  • Determine ALIGNMENT more cleverly and move it to util.h
  • Implement manual_malloc() helper function
• Add size constants for preallocated memory
• Switch to a single malloc call
• Support cloning a context into preallocated memory
• Check arguments of _preallocated functions
• Add tests for contexts in preallocated memory
• Export _preallocated functions
• Move _preallocated functions to separate header
• Explain caller's obligations for preallocated memory

This is a backport of secp256k1 PR566

Depends on D5030

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5031