Followups to #385 #390

Followups to #385:

• remove the empty src/jet/environment.rs file
• When creating tx environments we should assert that the utxo slice length matches the tx input length (comment is about bitcoin but we should do the same for elements) and add a # Panics doc about it.
• the get_annex method in src/jet/bitcoin/c_env.rs has a couple bugs 1 2 (probably also affects Elements; we should abstract and share this code between the environments)