add test to demonstrate pair of "half" kernels sharing same public excess (#3314)

* cleanup how we handle key splitting for transaction offset
add test to demonstrate a pair of transaction halves sharing same kernel excess

* cleanup

* cleanup

pool/src/pool.rs

@@ -29,6 +29,7 @@ use grin_util as util;
 use std::cmp::Reverse;
 use std::collections::{HashMap, HashSet};
 use std::sync::Arc;
+use util::static_secp_instance;
 
 pub struct Pool<B, V>
 where
@@ -272,7 +273,12 @@ where
 		header: &BlockHeader,
 	) -> Result<BlockSums, PoolError> {
 		let overage = tx.overage();
-		let offset = (header.total_kernel_offset() + tx.offset.clone())?;
+
+		let offset = {
+			let secp = static_secp_instance();
+			let secp = secp.lock();
+			header.total_kernel_offset().add(&tx.offset, &secp)
+		}?;
 
 		let block_sums = self.blockchain.get_block_sums(&header.hash())?;