use scl.to_table() for action and rules headers

[naturallymitchell]

https://github.com/foundpatterns/lighttouch-base/blob/f8d1e9e81c97e0c877aca0eba4c61b1ba08b63de/loaders/events.lua#L27-L30
looks like this is a reusable function.. list to table? table_ext.from_list() ?

https://github.com/foundpatterns/lighttouch-base/search?utf8=%E2%9C%93&q=lines&type=
looks like it should be yaml to_table and

[naturallymitchell]

Arnaud made an issue about parsing action and rule headers "the same ways as parsing documents"
not before the empty line, but before the spec's own ... or --- separater
parse those lines in yaml_to_table

[naturallymitchell]

use content.split_header()

#18