add resizeable scratch space API

Alignment support by Pieter Wuille.

src/scratch.h

@@ -0,0 +1,35 @@
+/**********************************************************************
+ * Copyright (c) 2017 Andrew Poelstra	                              *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_SCRATCH_
+#define _SECP256K1_SCRATCH_
+
+/* The typedef is used internally; the struct name is used in the public API
+ * (where it is exposed as a different typedef) */
+typedef struct secp256k1_scratch_space_struct {
+    void *data;
+    size_t offset;
+    size_t init_size;
+    size_t max_size;
+    const secp256k1_callback* error_callback;
+} secp256k1_scratch;
+
+static secp256k1_scratch* secp256k1_scratch_create(const secp256k1_callback* error_callback, size_t init_size, size_t max_size);
+static void secp256k1_scratch_destroy(secp256k1_scratch* scratch);
+
+/** Returns the maximum allocation the scratch space will allow */
+static size_t secp256k1_scratch_max_allocation(const secp256k1_scratch* scratch, size_t n_objects);
+
+/** Attempts to allocate so that there are `n` available bytes. Returns 1 on success, 0 on failure */
+static int secp256k1_scratch_resize(secp256k1_scratch* scratch, size_t n, size_t n_objects);
+
+/** Returns a pointer into the scratch space or NULL if there is insufficient available space */
+static void *secp256k1_scratch_alloc(secp256k1_scratch* scratch, size_t n);
+
+/** Resets the returned pointer to the beginning of space */
+static void secp256k1_scratch_reset(secp256k1_scratch* scratch);
+
+#endif