MessageParser::get should handle variants specially

#82 brought to light that variants are not very userfriendly

* Either `get()` could check if the next value is a variant and do "the right thing" but I'll have to think about whether this has any edge cases that would break if I do this.
* Or I could introduce a get_variant() that works similarly to `get()` which essentially wraps the `get::()?.get::()`

In any case, users should rarely have to actually call `get::()`

Start by reading MessageParser::get and the variant behavior exposed by issue #82. Decide between special handling in get() and a separate get_variant() API; done means users rarely need to call get::() directly and the chosen behavior covers relevant edge cases.

Written by the indexing model from the issue text.