feat(ppx): disallow attribute payload in [@mel.new]

[anmonteiro]

this change unifies the FFI language a bit more, disallowing external declarations with [@@mel.new "Constructor"].

it plays better with the warnings that we issue when the external primitive name is "". In this case, the constructor must always appear in the primitive name, which makes the FFI language a bit more consistent.

[jchavarri]

Looks great. I wonder though if there is any case / weird combination of attributes that could allow to express something with the payload that is not possible to express with prim string? 🤔 Hopefully not.

[anmonteiro]

I wonder though if there is any case / weird combination of attributes that could allow to express something with the payload that is not possible to express with prim string?

I don't think so. The only ones that come to mind are @mel.module and @mel.scope, which both carry payload.