Add trait import and trait implementation contracts

This commit adds the following to the example project:

- `ft-transfer-many`: A contract that imports a trait and references it in a
function parameter.
- `rendezvous-token`: A contract that implements the referenced trait.

Additionally, it includes an invariant and a property test for the
ft-transfer-many contract to demonstrate the random selection of trait
implementations in action.

example/Clarinet.toml

@@ -6,6 +6,14 @@ cache_dir = "./.cache"
 [[project.requirements]]
 contract_id = 'SP4SZE494VC2YC5JYG7AYFQ44F5Q4PYV7DVMDPBG.ststx-token'
 
+[[project.requirements]]
+contract_id = 'SP4SZE494VC2YC5JYG7AYFQ44F5Q4PYV7DVMDPBG.sip-010-trait-ft-standard'
+
+[contracts.rendezvous-token]
+path = 'contracts/rendezvous-token.clar'
+clarity_version = 2
+epoch = 2.5
+
 [contracts.cargo]
 path = "contracts/cargo.clar"
 clarity_version = 3
@@ -26,8 +34,8 @@ path = "contracts/slice.clar"
 clarity_version = 3
 epoch = 3.0
 
-[contracts.trait]
-path = 'contracts/trait.clar'
+[contracts.ft-transfer-many]
+path = 'contracts/ft-transfer-many.clar'
 clarity_version = 3
 epoch = 3.0
 

example/contracts/ft-transfer-many.clar

@@ -0,0 +1,21 @@
+(use-trait ft-trait 'SP4SZE494VC2YC5JYG7AYFQ44F5Q4PYV7DVMDPBG.sip-010-trait-ft-standard.sip-010-trait)
+
+(define-public (transfer-many
+    (transfers-list
+      (list 5 {token: <ft-trait>, recipient: principal, amount: uint})
+    )
+  )
+  (ok (map transfer transfers-list))
+)
+
+(define-private (transfer
+    (one-transfer {token: <ft-trait>, recipient: principal, amount: uint})
+  )
+  (let
+    (
+      (ft (get token one-transfer))
+      (recipient (get recipient one-transfer))
+      (amount (get amount one-transfer))
+    )
+    (contract-call? ft transfer amount tx-sender recipient none))
+)

example/contracts/ft-transfer-many.tests.clar

@@ -0,0 +1,47 @@
+;; Invariants
+
+(define-read-only (invariant-trait (address principal))
+  (let
+    (
+      (total-supply
+        (unwrap-panic (contract-call? .rendezvous-token get-total-supply)))
+      (user-balance
+        (unwrap-panic
+          (contract-call?
+            .rendezvous-token
+            get-balance
+            address
+          )
+        )
+      )
+    )
+    (<= user-balance total-supply)
+  )
+)
+
+;; Properties
+
+(define-public (test-transfer
+    (token <ft-trait>) (recipient principal) (amount uint)
+  )
+  (let
+    (
+      (sender-balance-before
+        (unwrap-panic (contract-call? token get-balance tx-sender)))
+      (transfer-result
+        (transfer {token: token, recipient: recipient, amount: amount}))
+      (sender-balance-after
+        (unwrap-panic (contract-call? token get-balance tx-sender)))
+    )
+    (match transfer-result
+      result
+        (ok
+          (is-eq
+            sender-balance-after
+            (- sender-balance-before amount)
+          )
+        )
+      error (ok false)
+    )
+  )
+)

example/contracts/rendezvous-token.clar

@@ -0,0 +1,41 @@
+(impl-trait 'SP4SZE494VC2YC5JYG7AYFQ44F5Q4PYV7DVMDPBG.sip-010-trait-ft-standard.sip-010-trait)
+
+(define-fungible-token rendezvous)
+
+;; SIP-010 methods.
+
+(define-read-only (get-total-supply)
+  (ok (ft-get-supply rendezvous))
+)
+
+(define-read-only (get-name)
+  (ok "Rendezvous Token")
+)
+
+(define-read-only (get-symbol)
+  (ok "rendezvous")
+)
+
+(define-read-only (get-decimals)
+  (ok u6)
+)
+
+(define-read-only (get-balance (account principal))
+  (ok (ft-get-balance rendezvous account))
+)
+
+(define-read-only (get-token-uri)
+  (ok (some u""))
+)
+
+(define-public (transfer
+    (amount uint)
+    (sender principal)
+    (recipient principal)
+    (memo (optional (buff 34)))
+  )
+  (match (ft-transfer? rendezvous amount sender recipient)
+    response (ok response)
+    error (err error)
+  )
+)