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/bench/index_blockfilter.cpp

@@ -25,6 +25,8 @@
 #include <memory>
 #include <vector>
 
+using namespace util::hex_literals;
+
 // Very simple block filter index sync benchmark, only using coinbase outputs.
 static void BlockFilterIndexSync(benchmark::Bench& bench)
 {