Introduce PackageTemplae, a replacement of InputMaterial

PackageTemplate aims to replace InputMaterial, introducing a clean
interface to manipulate a wide range of claim types without
OnchainTxHandler aware of special content of each.

This is used in next commits.

Author: Antoine Riard

lightning/src/ln/chan_utils.rs

@@ -40,7 +40,7 @@ use core::ops::Deref;
 use chain;
 
 // Maximum size of a serialized HTLCOutputInCommitment
-const HTLC_OUTPUT_IN_COMMITMENT_SIZE: usize = 1 + 8 + 4 + 32 + 5;
+pub(crate) const HTLC_OUTPUT_IN_COMMITMENT_SIZE: usize = 1 + 8 + 4 + 32 + 5;
 
 pub(crate) const MAX_HTLCS: u16 = 483;