Catch up the consequences of Number changes

- Change the Number::maxIntValue to all 9's.
- Add integral() to Asset (copied from Lending)
- Add toNumber() functions to STAmount, MPTAmount, XRPAmount to allow
  explicit conversions with enforcement options.
- Add optional Number::EnforceInteger options to STAmount and STNumber
  ctors, conversions, etc. IOUs are never checked.
- Update Vault transactors, and helper functions, to check restrictions.
- Fix and add Vault tests.

src/test/basics/Number_test.cpp

@@ -858,7 +858,7 @@ public:
             {
                 BEAST_EXPECT(e.what() == "Number::operator= integer overflow"s);
                 // The throw is done _after_ the number is updated.
-                BEAST_EXPECT((a == Number{2, 14}));
+                BEAST_EXPECT((a == Number::maxIntValue * 2));
                 BEAST_EXPECT(!a.valid());
             }
             try
@@ -870,7 +870,7 @@ public:
             {
                 BEAST_EXPECT(e.what() == "Number::Number integer overflow"s);
                 // The Number doesn't get updated because the ctor throws
-                BEAST_EXPECT((a == Number{2, 14}));
+                BEAST_EXPECT((a == Number::maxIntValue * 2));
                 BEAST_EXPECT(!a.valid());
             }
             a = Number(1, 10);