Add function syntax support in `plain-let`
Not sure we want to go down that route (next comes `define-operator`, then declaration macros, ...).
I think plain-let is only really useful for machine-generated code (typically macros), so we might as well keep it simple.
+%%%% (currently handle definition with `_=_`) +%%%% (would declaration be useful ? (e.g. `_:_`))
Type declarations should never be needed, AFAIK.
Stefan