Merge #35: Nodes

637576b Rename Type::ty (Christian Lewe)
7649242 Remove #[allow(dead_code)] (Christian Lewe)
29cec70 Adapt policy compiler (Christian Lewe)
bf875cf Adapt test programs (Christian Lewe)
cec3081 Add decoding of programs (Christian Lewe)
e0571aa Implement finalization (Christian Lewe)
cd14fbc Adapt Bit Machine (Christian Lewe)
ff8b92f Adapt decoding (Christian Lewe)
7fe81f1 Adapt inference (Christian Lewe)
a97ba40 Adapt analysis (Christian Lewe)
2d8a66a Adapt DagIterable (Christian Lewe)
f28d109 Add Node (Christian Lewe)
18410e2 Add CommitNode (Christian Lewe)
2f6412a Remove linear programs (Christian Lewe)
33e2a9c Temporarily remove fuzz tests (Christian Lewe)

Pull request description:

  This PR removes linear programs entirely and replaces them with heap structures.

  # Philosophy

  Linear programs exist because of the (de)serialisation of Simplicity programs. However, besides that there is really no need for index bookkeeping in our code. On top of that, sharing and pruning can are transport-layer optimisations and should be done during serialisation. Therefore, let us use one of Rust's strengths instead, namely reference bookkeeping.

  # Immediate effects

  The code is more concise and readable. Children of left and right assertions are checked to be hidden nodes _(which already gave away a faulty program)_. Providing a witness during finalisation gives proper errors and never panics. Fuzz tests have been temporarily removed, because programs can no longer be encoded (future PR).

  # Future extensions

  By constructing programs inductively, we enable many useful features: We could locally check if the types of children nodes match the type of their parent as we construct it, and provide an error to the user if there is a mismatch. We could implement a program factory that keeps track of the sub-programs it has produced and that returns an `Rc::clone` whenever the same sub-program is produced twice. In general, freeing ourselves from indices allows for many runtime optimisations in Rust.

ACKs for top commit:
  apoelstra:
    ACK 637576b

Tree-SHA512: 935fc52052dfcf207b316e0a997e44254c3ab1ac26aa24a529c414882ca975f725d23fb62eec716c7baa563dc352ccc92d0624196db96f70314204ecb145eaa5

Author: apoelstra

fuzz/Cargo.toml

@@ -21,19 +21,3 @@ members = ["."]
 [[bin]]
 name = "rtt_natural"
 path = "fuzz_targets/rtt_natural.rs"
-
-[[bin]]
-name = "rtt_program"
-path = "fuzz_targets/rtt_program.rs"
-
-[[bin]]
-name = "rtt_witness"
-path = "fuzz_targets/rtt_witness.rs"
-
-[[bin]]
-name = "rtt_program_witness"
-path = "fuzz_targets/rtt_program_witness.rs"
-
-[[bin]]
-name = "rtt_dag"
-path = "fuzz_targets/rtt_dag.rs"