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/STAmount.cpp

@@ -550,20 +550,7 @@ STAmount::getFullText() const
     std::string ret;
 
     ret.reserve(64);
-    ret = getText() + "/" + to_string(mIssue.currency);
-
-    if (!mIsNative)
-    {
-        ret += "/";
-
-        if (isXRP(*this))
-            ret += "0";
-        else if (mIssue.account == noAccount())
-            ret += "1";
-        else
-            ret += to_string(mIssue.account);
-    }
-
+    ret = getText() + "/" + mIssue.getText();
     return ret;
 }