Support '+' operator on strings / printf macro

[matiwinnetou]

This feels clunky to write:

        trace append_string(@"on:", cbor.diagnostic(on_chain_results_hash))
        trace append_string(@"off:", cbor.diagnostic(off_chain_batch_hash))

        trace append_string(
          @"results:",
          from_int(list.length(on_chain_results)),
        )

Wouldn't it be better and smoother for a developer to write:

trace @"on: " + cbor.diagnostic(on_chain_results_hash)

alternatively support something like println or formatf

trace @formatf("on:%s", cbor.diagnostic(on_chain_results_hash))

Much better, of course ideal would be string interpolation but I guess we are far from this one.