scratch: rename max_size to size, document that extra will actually be allocated

Author: apoelstra

include/secp256k1.h

@@ -285,11 +285,12 @@ SECP256K1_API void secp256k1_context_set_error_callback(
  *
  *  Returns: a newly created scratch space.
  *  Args: ctx:  an existing context object (cannot be NULL)
- *  In:   max_size: maximum amount of memory to allocate
+ *  In:   size: amount of memory to be available as scratch space. Some extra
+ *              (<100 bytes) will be allocated for extra accounting.
  */
 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT secp256k1_scratch_space* secp256k1_scratch_space_create(
     const secp256k1_context* ctx,
-    size_t max_size
+    size_t size
 ) SECP256K1_ARG_NONNULL(1);
 
 /** Destroy a secp256k1 scratch space.

src/scratch_impl.h

@@ -10,15 +10,15 @@
 #include "util.h"
 #include "scratch.h"
 
-static secp256k1_scratch* secp256k1_scratch_create(const secp256k1_callback* error_callback, size_t max_size) {
+static secp256k1_scratch* secp256k1_scratch_create(const secp256k1_callback* error_callback, size_t size) {
     const size_t base_alloc = ((sizeof(secp256k1_scratch) + ALIGNMENT - 1) / ALIGNMENT) * ALIGNMENT;
-    void *alloc = checked_malloc(error_callback, base_alloc + max_size);
+    void *alloc = checked_malloc(error_callback, base_alloc + size);
     secp256k1_scratch* ret = (secp256k1_scratch *)alloc;
     if (ret != NULL) {
         memset(ret, 0, sizeof(*ret));
         memcpy(ret->magic, "scratch", 8);
         ret->data = (void *) ((char *) alloc + base_alloc);
-        ret->max_size = max_size;
+        ret->max_size = size;
     }
     return ret;
 }