variant: visit lambda and value-initialize by index

[jeffro256]

Two new features for tools::variant:

• Visit by lambda instead of static visitor (possible now because result type is deduced)
• Value-initialize the variant based on a type index

[vtnerd]

In what situations would you need to construct based on index rather than type?

[jeffro256]

deserialization

Edit: more specifically for deserialization of variants which have complex logic differing between the types. Instead of doing the logic at a high level, then assigning the variant, you can break down the process into independent chunks. When you de-serialize a selector, you can apply the selector (in this case value initializing the variant), then fire away and let the specific code for that variant handle the logic. In my opinion, this leads to much cleaner variant de-serialization code when the variant is non-trivial.

[jeffro256]

Something similar is already done in the current variant de serialization code:

monero/src/serialization/variant.h

Lines 74 to 80 in 059028a

	current_type x;
	if(!do_serialize(ar, x))
	{
	ar.set_fail();
	return false;
	}
	v = x;