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.

include/xrpl/protocol/STNumber.h

@@ -56,6 +56,18 @@ public:
     bool
     isDefault() const override;
 
+    /// Sets the flag on the underlying number
+    void
+    setIntegerEnforcement(Number::EnforceInteger enforce);
+
+    /// Gets the flag value on the underlying number
+    Number::EnforceInteger
+    integerEnforcement() const noexcept;
+
+    /// Checks the underlying number
+    bool
+    valid() const noexcept;
+
     operator Number() const
     {
         return value_;