Merge #566: Enable context creation in preallocated memory

0522caa Explain caller's obligations for preallocated memory (Tim Ruffing)
238305f Move _preallocated functions to separate header (Tim Ruffing)
695feb6 Export _preallocated functions (Tim Ruffing)
814cc78 Add tests for contexts in preallocated memory (Tim Ruffing)
ba12dd0 Check arguments of _preallocated functions (Tim Ruffing)
5feadde Support cloning a context into preallocated memory (Tim Ruffing)
c4fd5da Switch to a single malloc call (Tim Ruffing)
ef020de Add size constants for preallocated memory (Tim Ruffing)
1bf7c05 Prepare for manual memory management in preallocated memory (Tim Ruffing)

Pull request description:

  @apoelstra

  This builds on #557.

  Manually managing memory is always a pain in the ass in some way. I tried to keep the pain manageable. I'm open to suggestions to make this less ugly or error-prone.

  to do:
   * tests
   * export functions

ACKs for commit 0522ca:

Tree-SHA512: 8ddb5b70219b6f095e780a9812d2387ab2a7f399803ce4101e27da504b479a61ebe08b6380568c7ba6f1e73d7d0b1f58a3c0a66fa0fdec7a64cd0740e156ce38