avoid copying action when computing action digest

[spoonincode]

I was doing some research in this area and noticed that generate_action_digest() effectively copies an action's input and output (return) data. This is a refactor that avoids that. Discussion on removing checktime at #1411 (comment). Not really expecting a notable perf improvement but a cleanup nonetheless.

[greg7mdp]

Just noticed that, unfortunately, is_trivial_array won't be true for Hash members, so we won't benefit of the optimization in:

spring/libraries/libfc/include/fc/io/raw.hpp

Lines 672 to 676 in 436e188

	template<typename Stream, typename T, std::size_t S>
	inline auto pack( Stream& s, const std::array<T, S>& value ) -> std::enable_if_t<is_trivial_array<T>>
	{
	s.write((const char*)value.data(), S * sizeof(T));
	}

The previous code was iterating over the hashes anyway, so this version is not any slower, but:

1. the name is_trivial_array doesn't seem right to me, should probably be is_trivial_array_element
2. even better, just use: std::is_trivially_copyable_v<T>

I'm happy to make this change in another PR if you guys think it is worthwhile.

[spoonincode]

It does seem like is_trivially_copyable_v would be the right choice. But it seems more appropriate to touch raw.hpp in a different PR, if you want to take that on @greg7mdp

[greg7mdp]

It does seem like is_trivially_copyable_v would be the right choice. But it seems more appropriate to touch raw.hpp in a different PR, if you want to take that on @greg7mdp

Will do, creating an issue, thanks!