Introduce AMM support (XLS-30d): (#4294)

Add AMM functionality:
- InstanceCreate
- Deposit
- Withdraw
- Governance
- Auctioning
- payment engine integration

To support this functionality, add:
- New RPC method, `amm_info`, to fetch pool and LPT balances
- AMM Root Account
- trust line for each IOU AMM token
- trust line to track Liquidity Provider Tokens (LPT)
- `ltAMM` object

The `ltAMM` object tracks:
- fee votes
- auction slot bids
- AMM tokens pair
- total outstanding tokens balance
- `AMMID` to AMM `RootAccountID` mapping

Add new classes to facilitate AMM integration into the payment engine.
`BookStep` uses these classes to infer if AMM liquidity can be consumed.

The AMM formula implementation uses the new Number class added in #4192.
IOUAmount and STAmount use Number arithmetic.

Add AMM unit tests for all features.

AMM requires the following amendments:
- featureAMM
- fixUniversalNumber
- featureFlowCross

Notes:
- Current trading fee threshold is 1%
- AMM currency is generated by: 0x03 + 152 bits of sha256{cur1, cur2}
- Current max AMM Offers is 30

---------

Co-authored-by: Howard Hinnant <howard.hinnant@gmail.com>

src/ripple/protocol/impl/STVar.cpp

@@ -25,6 +25,7 @@
 #include <ripple/protocol/STBitString.h>
 #include <ripple/protocol/STBlob.h>
 #include <ripple/protocol/STInteger.h>
+#include <ripple/protocol/STIssue.h>
 #include <ripple/protocol/STObject.h>
 #include <ripple/protocol/STPathSet.h>
 #include <ripple/protocol/STVector256.h>
@@ -157,6 +158,9 @@ STVar::STVar(SerialIter& sit, SField const& name, int depth)
         case STI_ARRAY:
             construct<STArray>(sit, name, depth);
             return;
+        case STI_ISSUE:
+            construct<STIssue>(sit, name);
+            return;
         default:
             Throw<std::runtime_error>("Unknown object type");
     }
@@ -212,6 +216,9 @@ STVar::STVar(SerializedTypeID id, SField const& name)
         case STI_ARRAY:
             construct<STArray>(name);
             return;
+        case STI_ISSUE:
+            construct<STIssue>(name);
+            return;
         default:
             Throw<std::runtime_error>("Unknown object type");
     }