Simplify & modernize code:

- Simplify and consolidate code for parsing hex input.
- Replace beast::endian::order with boost::endian::order.
- Simplify CountedObject code.
- Remove pre-C++17 workarounds in favor of C++17 based solutions.
- Improve `base_uint` and simplify its hex-parsing interface by
  consolidating the `SexHex` and `SetHexExact` methods into one
  API: `parseHex` which forces callers to verify the result of
  the operation; as a result some public-facing API endpoints
  may now return errors when passed values that were previously
  accepted.
- Remove the simple fallback implementations of SHA2 and RIPEMD
  introduced to reduce our dependency on OpenSSL. The code is
  slow and rarely, if ever, exercised and we rely on OpenSSL
  functionality for Boost.ASIO as well.

src/test/rpc/Feature_test.cpp

@@ -129,7 +129,7 @@ class Feature_test : public beast::unit_test::suite
              ++it)
         {
             uint256 id;
-            id.SetHexExact(it.key().asString().c_str());
+            (void)id.parseHex(it.key().asString().c_str());
             if (!BEAST_EXPECT((*it).isMember(jss::name)))
                 return;
             bool expectEnabled = env.app().getAmendmentTable().isEnabled(id);