Pretty printer now prints grouped elements in the same line, closes #1383

This implementation slightly changes the semantic of Lindig's algorithm
to allow elements that belong to the same group to be printed together
in the same line, even if they do not fit the line fully. This was achieved
by changing `:break` to mean a possible break and `:flat` to force a flat
structure. Then deciding if a break works as a newline is just a matter
of checking if we have enough space until the next break that is not
inside a group (which is still flat).