Expand Number to support the full integer range (#6025)

- Refactor Number internals away from int64 to uint64 & a sign flag
  - ctors and accessors use `rep`. Very few things expose
    `internalrep`.
  - An exception is "unchecked" and the new "normalized", which explicitly
    take an internalrep. But with those special control flags, it's easier
    to distinguish and control when they are used.

- For now, skip the larger mantissas in AMM transactions and tests

- Remove trailing zeros from scientific notation Number strings
  - Update tests. This has the happy side effect of making some of the string
    representations _more_ consistent between the small and large
    mantissa ranges.

- Add semi-automatic rounding of STNumbers based on Asset types
  - Create a new SField metadata enum, sMD_NeedsAsset, which indicates
    the field should be associated with an Asset so it can be rounded.
  - Add a new STTakesAsset intermediate class to handle the Asset
    association to a derived ST class. Currently only used in STNumber,
    but could be used by other types in the future.
  - Add "associateAsset" which takes an SLE and an Asset, finds the
    sMD_NeedsAsset fields, and associates the Asset to them. In the case
    of STNumber, that both stores the Asset, and rounds the value
    immediately.
  - Transactors only need to add a call to associateAsset _after_ all of
    the STNumbers have been set. Unfortunately, the inner workings of
    STObject do not do the association correctly with uninitialized
    fields.
  - When serializing an STNumber that has an Asset, round it before
    serializing.
  - Add an override of roundToAsset, which rounds a Number value in place
    to an Asset, but without any additional scale.
  - Update and fix a bunch of Loan-related tests to accommodate the
    expanded Number class.

---------

Co-authored-by: Vito <5780819+Tapanito@users.noreply.github.com>

src/test/app/MPToken_test.cpp

@@ -1594,7 +1594,7 @@ class MPToken_test : public beast::unit_test::suite
             jv[jss::secret] = alice.name();
             jv[jss::tx_json] = pay(alice, bob, mpt);
             jv[jss::tx_json][jss::Amount][jss::value] =
-                to_string(maxMPTokenAmount + 1);
+                std::to_string(maxMPTokenAmount + 1);
             auto const jrr = env.rpc("json", "submit", to_string(jv));
             BEAST_EXPECT(jrr[jss::result][jss::error] == "invalidParams");
         }
@@ -2474,7 +2474,7 @@ class MPToken_test : public beast::unit_test::suite
                 alice.name(), makeMptID(env.seq(alice), alice));
 
             Json::Value jv = claw(alice, mpt(1), bob);
-            jv[jss::Amount][jss::value] = to_string(maxMPTokenAmount + 1);
+            jv[jss::Amount][jss::value] = std::to_string(maxMPTokenAmount + 1);
             Json::Value jv1;
             jv1[jss::secret] = alice.name();
             jv1[jss::tx_json] = jv;