refactor: Join-split example Part 1 (#4965)

The first of a two-part series to refactor the join-split example, the
goal being to focus the tests on circuit writing (no proof
construction), update them to reflect our newer idioms, and relocate
them. Key changes
1) Keep a subset of tests relating to circuit writing.
1) Remove code that has to do with proof construction.
1) Add a new circuit change detection paradigm: rather than hashing a
verification key (a proving system-dependent object), simply hash the
circuit-defining data in the circuit builder.
1) Delete unused code (some unused before, some unused now). 

The next PR AztecProtocol/aztec-packages#5016
will move and condense files, and update namespaces.

barretenberg.code-workspace

@@ -79,6 +79,9 @@
     // Some settings can only be configured here.
     // The following are just provided as example.
     "settings": {
+        "files.associations": {
+            "*.tcc": "cpp",
+          },
         //
         // Clangd. Note that this setting may be overridden by  user settings
         // to the default value "clangd".

cpp/scripts/bb-tests.sh

@@ -33,7 +33,6 @@ TESTS=(
   crypto_schnorr_tests
   crypto_sha256_tests
   ecc_tests
-  join_split_example_proofs_inner_proof_data_tests
   join_split_example_proofs_notes_tests
   numeric_tests
   plonk_tests

cpp/src/barretenberg/crypto/schnorr/schnorr.tcc

@@ -88,7 +88,7 @@ schnorr_signature schnorr_construct_signature(const std::string& message, const
     // method is overloaded to utilise a suitable entropy source
     // (see https://github.com/WebAssembly/WASI/blob/main/phases/snapshot/docs.md)
     //
-    // TODO: securely erase `k`
+    // TODO(https://github.com/AztecProtocol/barretenberg/issues/895): securely erase `k`
     Fr k = Fr::random_element();
 
     typename G1::affine_element R(G1::one * k);