StatePointer (#4074)

* Introducing linear pointer.

* Added design document.
Added StatePointer interface.
Updated design document.
Updated StatePointer implementation.
Added doc section for state pointer.

* Updated design document.
Added API for StatePointer.

* Update core/src/main/kotlin/net/corda/core/contracts/Structures.kt

Co-Authored-By: roger3cev <roger.willis@r3cev.com>

* Update core/src/main/kotlin/net/corda/core/contracts/Structures.kt

Co-Authored-By: roger3cev <roger.willis@r3cev.com>

* Update core/src/main/kotlin/net/corda/core/contracts/Structures.kt

Co-Authored-By: roger3cev <roger.willis@r3cev.com>

* Update docs/source/design/linear-pointer/design.md

Co-Authored-By: roger3cev <roger.willis@r3cev.com>

* Update docs/source/design/linear-pointer/design.md

Co-Authored-By: roger3cev <roger.willis@r3cev.com>

* Resolve pointers
Added test to check pointers are resolved.
Updated docs and kdocs.
Reverted changes to api-current.txt
Revert "Reverted changes to api-current.txt"
This reverts commit dc1cece91a595a4e772f63917b830c7e1fd0586d.
Fix CI bug.
Made StatePointers type safe.
Resolving StatePointers is now optionally recursive
Addressed review comments.

Fixed compile error.

Addressed review comments.

Fixed bug in state pointer search.
Improved efficiency of state pointer search.

Removed whitespace.

TxBuilder logs warning when no service hub is supplied for resolving pointers as opposed to throwing an exception.

* Addressed review comments.

samples/trader-demo/src/integration-test/kotlin/net/corda/traderdemo/TraderDemoTest.kt

@@ -40,6 +40,7 @@ class TraderDemoTest {
                     startNode(providedName = DUMMY_BANK_B_NAME, rpcUsers = listOf(demoUser)),
                     startNode(providedName = BOC_NAME, rpcUsers = listOf(bankUser))
             ).map { (it.getOrThrow() as InProcess) }
+
             val (nodeARpc, nodeBRpc) = listOf(nodeA, nodeB).map {
                 val client = CordaRPCClient(it.rpcAddress)
                 client.start(demoUser.username, demoUser.password).proxy