Replace base_uint(string) with from_hex_text<> (RIPD-897)

Removes the base_uint constructor that took a string.  Replaces
that functionality with two free functions named from_hex_text<>.
Use of from_hex_text<> looks like this:

auto v = from_hex_text<uint256>("AAA555");
static_assert (std::is_same<decltype(v), uint256>::value, "Huh!");

from_hex_text<> only operates on base_uint types.  At the moment the
list of those types include:

 o uint128,
 o uint160,
 o uint256,
 o Directory,
 o Account,
 o Currency, and
 o NodeID.

Using from_hex_text<> with any other types will not compile due to
an enable_if.

src/ripple/protocol/impl/Indexes.cpp

@@ -19,6 +19,7 @@
 
 #include <BeastConfig.h>
 #include <ripple/protocol/Indexes.h>
+#include <beast/utility/static_initializer.h>
 
 namespace ripple {
 
@@ -168,8 +169,10 @@ getQualityIndex (uint256 const& uBase, const std::uint64_t uNodeDir)
 uint256
 getQualityNext (uint256 const& uBase)
 {
-    static uint256 uNext ("10000000000000000");
-    return uBase + uNext;
+    static beast::static_initializer<uint256> const uNext (
+        from_hex_text<uint256>("10000000000000000"));
+
+    return uBase + *uNext;
 }
 
 std::uint64_t