Change set_-functions behaviour to not create intermediate structures.

[blackwinter]

add_field() and set_field() are going to be equivalent with #308, but according to Catmandu's documentation set_field() "will not create the intermediate structures if they are missing".

Functional review: @TobiasNx
Code review: @blackwinter / @fsteeg

[TobiasNx]

The aspect of the intermediate structure seems to be true for all fixes of the set_-family so not just for set_field but also for set_array and set_hash. I think that the adjustment for all these fixes makes sense since we want to adjust to the behaviour of Catmandu BUT on the side of lobid and oersi we rely on a fixes that create empty hashes and empty arrays WITH the intermediate structure. So If we move on with this, we should also introduce two new fix functions add_array and add_hash as counterpart to set_hash and set_array.

PS: The new functions add_hash and add_array could reuse the current implementation of set_array and set_hash while the version without creating intermediate structure would replace the the current one.

[TobiasNx]

add_array and add_hash are introduced now and the set functions are only aliases now.
We should advise anybody to use the add functions instead of the set ones until the new behaviour is introduced.

Also wenn the behaviour change is implemented we should rename all set_ tests to add_ ones as well as add new set functions.