Improve error reporting and improve docs as suggested by Andy, and
adjust `conv` -> `conv*` to fit a naming convention.
original commit: b34817aea5d3c4862e7bb313ee9f5281472a832f
Also, report an error if a string type is misused as an argument (for
foreign procedures) or result (for foreign callables) with
`__collect_safe`.
original commit: cdbfa3d86cb0719bf0979b3fe0aa5c4383282b77
