While working on the full WASI spec, it turned out that we need two
tweaks to `GuestError`:
1. we need to support conversion from `TryFromIntError`, and
2. we need to invoke `e.into()` when unwrapping the result of `try_into()`
in auto-implementation of raw interface functions.
Both problems seem to originate for "transparent" builtin types since
for those we don't really provide a `TryFrom` implementation like for
compound types, e.g., enums, flags, etc.
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