Consistently treat `_` as unused in function argument lists

[Keno]

Since assignment to _ is disallowed, it should be consistently treated as unused in an argument list. We do this for plain definitions, but accidentally try to assign to it when generating wrappers:

julia> f(_) = 1
f (generic function with 1 method)

julia> f(_; a) = 1
ERROR: syntax: all-underscore identifiers are write-only and their values cannot be used in expressions around REPL[1]:1
Stacktrace:
 [1] top-level scope
   @ REPL[1]:1