feat(mmr::authentication_struct): Move witness generation from twenty-first to this repo

The witness generation is somewhat program-specific, so it seems to me
to be a better fit for this repo than for twenty-first.

Cf. Neptune-Crypto/twenty-first#228

Author: Sword-Smith

tasm-lib/src/mmr.rs

@@ -1,3 +1,4 @@
+pub mod authentication_struct;
 pub mod bag_peaks;
 pub mod calculate_new_peaks_from_append;
 pub mod calculate_new_peaks_from_leaf_mutation;

tasm-lib/src/mmr/authentication_struct.rs

@@ -0,0 +1 @@
+pub mod shared;