Introduce min/max observers for Number

Three static member functions are introduced with
definitions consistent with std::numeric_limits:

static constexpr Number min() noexcept;

  Returns: The minimum positive value.  This is the value closest to zero.

static constexpr Number max() noexcept;

  Returns: The maximum possible value.

static constexpr Number lowest() noexcept;

  Returns: The negative value which is less than all other values.

src/ripple/basics/Number.h

@@ -81,6 +81,13 @@ class Number
     Number&
     operator/=(Number const& x);
 
+    static constexpr Number
+    min() noexcept;
+    static constexpr Number
+    max() noexcept;
+    static constexpr Number
+    lowest() noexcept;
+
     explicit operator XRPAmount() const;  // round to nearest, even on tie
     explicit operator rep() const;        // round to nearest, even on tie
 
@@ -290,6 +297,24 @@ operator/(Number const& x, Number const& y)
     return z;
 }
 
+inline constexpr Number
+Number::min() noexcept
+{
+    return Number{minMantissa, minExponent, unchecked{}};
+}
+
+inline constexpr Number
+Number::max() noexcept
+{
+    return Number{maxMantissa, maxExponent, unchecked{}};
+}
+
+inline constexpr Number
+Number::lowest() noexcept
+{
+    return -Number{maxMantissa, maxExponent, unchecked{}};
+}
+
 inline constexpr bool
 Number::isnormal() const noexcept
 {

src/ripple/basics/impl/IOUAmount.cpp

@@ -53,7 +53,7 @@ IOUAmount::normalize()
 
     if (*stNumberSwitchover)
     {
-        const Number v{mantissa_, exponent_};
+        Number const v{mantissa_, exponent_};
         mantissa_ = v.mantissa();
         exponent_ = v.exponent();
         if (exponent_ > maxExponent)