MessageParser::get should handle variants specially

[KillingSpark]

#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::<Variant>()?.get::<MyType>()

In any case, users should rarely have to actually call get::<Variant>()