rangeproof: add secp256k1_rangeproof_max_size function to estimate rangeproof size

Provides a method that will give an upper bound on the size of a rangeproof,
given an upper bound on the value to be passed in and an upper bound on the
min_bits parameter.

There is a lot of design freedom here since the actual size of the rangeproof
depends on every parameter passed to rangeproof_sign, including the value to
be proven, often in quite intricate ways. For the sake of simplicity we assume
a nonzero `min_value` and that `exp` will be 0 (the default, and size-maximizing,
choice), and provide an exact value for a proof of the given value and min_bits.

include/secp256k1_rangeproof.h

@@ -287,6 +287,33 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_rangeproof_info(
   size_t plen
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
 
+/** Returns an upper bound on the size of a rangeproof with the given parameters
+ *
+ * An actual rangeproof may be smaller, for example if the actual value
+ * is less than both the provided `max_value` and 2^`min_bits`, or if
+ * the `exp` parameter to `secp256k1_rangeproof_sign` is set such that
+ * the proven range is compressed. In particular this function will always
+ * overestimate the size of single-value proofs. Also, if `min_value`
+ * is set to 0 in the proof, the result will usually, but not always,
+ * be 8 bytes smaller than if a nonzero value had been passed.
+ *
+ * The goal of this function is to provide a useful upper bound for
+ * memory allocation or fee estimation purposes, without requiring
+ * too many parameters be fixed in advance.
+ *
+ * To obtain the size of largest possible proof, set `max_value` to
+ * `UINT64_MAX` (and `min_bits` to any valid value such as 0).
+ *
+ *  In:       ctx: pointer to a context object
+ *      max_value: the maximum value that might be passed for `value` for the proof.
+ *       min_bits: the value that will be passed as `min_bits` for the proof.
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT size_t secp256k1_rangeproof_max_size(
+  const secp256k1_context* ctx,
+  uint64_t max_value,
+  int min_bits
+) SECP256K1_ARG_NONNULL(1);
+
 # ifdef __cplusplus
 }
 # endif