Merkle Sum Tree refactoring

[enricobottazzi]

The PR introduces some refactoring on the Merkle Sum Tree chip:

• Minor refactoring of the assing_leaf_hash_and_balance function in the MST chip
• Added support to circuit printing
• Refactoring of the enforce_less_than function in the MST chip
• Added more granular testing. In the previous version, the testing was only asserting for a generic error in the proof generation, now it is asserting for a specific error generation
• Added support, and related testing, for full_prover and full_verifier utils. These functions perform a full verification, simulating a production-ready environment. This is compared to all the other tests which are performed using the MockProver
• Moved merkle-sum-tree-rust from an external repository to an internal module a6cccb9
• Moved tests for merkle-sum-tree in a separate file 2dafd30
• Moved tests for circuits in a separate file 166588d
• Add feature to merkle-sum-tree constructor to prevent creating a tree that will overflow the prime db99ee2
• Added constants method for poseidon specifications dfc5d7e
• Removed reference to generic Field types in the library and replaced it with the actual one that we are gonna use, namely bn256 dfc5d7e
• Minor fixes in tests to support the slight modification of the circuit structure dfc5d7e
• Fixed swap constraint as it was over constraining the circuit 6c7d2f8

[sifnoc]

Looks good to me!
Please don't forget to handle https://github.com/summa-dev/merkle-sum-tree-rust after merging this.