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/test/app/CrossingLimits_test.cpp

@@ -18,31 +18,13 @@
 #include <ripple/beast/unit_test.h>
 #include <ripple/protocol/Feature.h>
 #include <test/jtx.h>
+#include <test/jtx/TestHelpers.h>
 
 namespace ripple {
 namespace test {
 
 class CrossingLimits_test : public beast::unit_test::suite
 {
-private:
-    void
-    n_offers(
-        jtx::Env& env,
-        std::size_t n,
-        jtx::Account const& account,
-        STAmount const& in,
-        STAmount const& out)
-    {
-        using namespace jtx;
-        auto const ownerCount = env.le(account)->getFieldU32(sfOwnerCount);
-        for (std::size_t i = 0; i < n; i++)
-        {
-            env(offer(account, in, out));
-            env.close();
-        }
-        env.require(owners(account, ownerCount + n));
-    }
-
 public:
     void
     testStepLimit(FeatureBitset features)