BREAKING: Refactor, drop config atom in favour of explicit `t` config arg (thanks asmala!)

A number of smaller changes were unfortunately merged into this commit. See
CHANGELOG for details.
Add `scope` fn for scope merging
Throw away recursive/chained translation aliases
Refactor `expand-dict` -> `inherit-parent-trs`
Allow `:missing` to use 3 input args separately
Add unit test for multiple invalid `t` fallbacks