Handle tacit functions

**Is your feature request related to a problem? Please describe.**
As a tacit programmer, I'd like to be able to save and retrieve tacit functions

**Describe the solution you'd like**
Allow `]add avg←+⌿÷≢` and store it in avg.aplf and detect that it isn't a dfn or tradfn when loading it back in.

**Describe alternatives you've considered**
Since the definition essentially needs to be executed to re-establish, maybe it belongs in avg.apls?