fix slurping into function definition

[simeonschaub]

Discovered while working on #40737. Currently, this throws an
unintuitive error:

julia> a, f()... = 1, 2, 3
ERROR: syntax: ssavalue with no def
Stacktrace:
 [1] top-level scope
   @ REPL[1]:1

Might as well fix this properly - Why not allow function definitions to
slurp a little from time to time? ;)

[simeonschaub]

Not claiming it's necessarily all that useful, but I think this is pretty cool at least:

julia> x = [1, 2, 3];

julia> _, f()... = x;

julia> f()
2-element Vector{Int64}:
 2
 3

julia> x[3] = 4;

julia> f()
2-element Vector{Int64}:
 2
 4

[simeonschaub]

Bump.

[vtjnash]

Seems unhappy with some merge conflict?

[simeonschaub]

Might be related to #40737, will look into it.

[simeonschaub]

Yes, the example linked above now doesn't close over x anymore, but instead the call to Base.rest now happens outside of f's body. I think with #40737 merged, this is actually the behavior we want.