RFC: Split the `||` unboxed closure form into two forms and remove the capture clauses and self type specifiers

Can this really not be inferred? With enough complexity in the compiler, it could default to always moving into closures unless the value moved into it is used after the closure is defined, in which case it could default to by-reference. Is the problem with this approach just that it would require an incredibly complex (and therefore probably buggy) implementation in the compiler, or is there another reason why this can’t be inferred?