Split block and tx context, use out parameter

[chfast]

This split transaction and block context into separate structs. They have different lifetimes so the Host can keep the same block context while processing multiple transactions.

We also return these by out-parameter because otherwise additional copy is needed in VM.

[yperbasis]

Probably unintentional change

[chfast]

This can be made to work as desired without without ABI change using C restrict. But only in to-be-released clang 14.

diff --git a/include/evmc/evmc.hpp b/include/evmc/evmc.hpp
index 2007b94..8655531 100644
--- a/include/evmc/evmc.hpp
+++ b/include/evmc/evmc.hpp
@@ -563,6 +563,11 @@ public:
         return result{host->call(context, &message)};
     }

+    void helper(evmc_tx_context& __restrict__ tx) const noexcept
+    {
+        tx = host->get_tx_context(context);
+    }
+
     /// @copydoc HostInterface::get_tx_context()
     ///
     /// The implementation caches the received transaction context
@@ -572,7 +577,7 @@ public:
     evmc_tx_context get_tx_context() const noexcept final
     {
         if (tx_context.block_timestamp == 0)
-            tx_context = host->get_tx_context(context);
+            helper(tx_context);
         return tx_context;
     }

So I plan to do the ABI change but only in the aspect of parameter passing. The split will not be done until I collect stats about transaction access usage.

[chfast]

For now #631 should be enough.