Truncate digest bits (#50)

* apply a hash function before adding to transcript

* truncate shape_digest into 250 bits

* add missing file

* fix clippy

* cargo fmt

src/constants.rs

@@ -0,0 +1,2 @@
+pub(crate) const NUM_CHALLENGE_BITS: usize = 128;
+pub(crate) const NUM_HASH_BITS: usize = 250;