Merge #523: scratch: add stack frame support

6fe5043 scratch: add stack frame support (Andrew Poelstra)

Pull request description:

  Replaces the single-blob stack space ith one that internally manages multiple blobs, which are exposed to the user as "frames". Users allocate new blobs with `secp256k1_scratch_allocate_frame` and deallocate them with `secp256k1_scratch_deallocate_frame`. Then any calls to `secp256k1_scratch_alloc` use the frame at the top of the stack. This is guaranteed to succeed, assuming that the frame allocation succeeded and that the user is not requesting more memory than the frame was allocated with.

Tree-SHA512: 0b2072c5b9df8f3b40fb6d76e94fcfcc6a03a7da33e31249b5f24b02eb8a3311f282f6a4732153d6101968de8f9a568009a72735a1cc688a0f3040055799a09d