Bring back `as_fn()` (function-declaration interpreter)

[egnha]

Bring back as_fn(), in modified form:

as_fn(x, y, ... ~ body)  # fn(x, y, ... ~ body)
as_fn(~ foo(.))          # fn(. ~ foo(.))
as_fn(..f = foo)         # foo
as_fn(..f = "foo")       # match.fun("foo")

(Backport to nofrills?)