refactor: Prepare for ParseHex -> ""_hex scripted-diff

- Adds using namespace.
- Extracts ToScript helper function from ScriptFromHex, to be used heavily in the next commit.
- Changes ScriptFromHex from using ParseHex to TryParseHex, now asserting the string is valid.
- Use even number of hex digits in comment (and apply replacement from next commit to only touch line once).

src/test/script_standard_tests.cpp

@@ -16,6 +16,7 @@
 
 #include <univalue.h>
 
+using namespace util::hex_literals;
 
 BOOST_FIXTURE_TEST_SUITE(script_standard_tests, BasicTestingSetup)