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/rpc/handlers/CanDelete.cpp

@@ -70,9 +70,9 @@ Json::Value doCanDelete (RPC::Context& context)
                     canDeleteStr.find_first_not_of("0123456789abcdef") ==
                     std::string::npos)
             {
-                uint256 ledgerHash (canDeleteStr);
-                Ledger::pointer ledger =
-                        context.netOps.getLedgerByHash (ledgerHash);
+                Ledger::pointer ledger = context.netOps.getLedgerByHash (
+                    from_hex_text<uint256>(canDeleteStr));
+
                 if (!ledger)
                     return RPC::make_error(rpcLGR_NOT_FOUND, "ledgerNotFound");
 

src/ripple/rpc/handlers/Tx.cpp

@@ -55,7 +55,8 @@ Json::Value doTx (RPC::Context& context)
     if (!isHexTxID (txid))
         return rpcError (rpcNOT_IMPL);
 
-    auto txn = getApp().getMasterTransaction ().fetch (uint256 (txid), true);
+    auto txn = getApp().getMasterTransaction ().fetch (
+        from_hex_text<uint256>(txid), true);
 
     if (!txn)
         return rpcError (rpcTXN_NOT_FOUND);