Consider providing version of to_native for select types that doesn't require type hints

Many types have a single "obvious" conversion to a native type. For example:

```
# No other conversion exists
x::U16 -> u16

# Most likely conversion to exist, since T might require
# conversion itself, making returning &[T] impossible
x::Slice -> Vec
```

It would be nice to provide these "obvious" conversions where possible without requiring type hints.

Start by locating the existing `to_native` implementation and its type-hint handling. Review how `U16` and `Slice` are currently converted, then define which conversions are unambiguous enough to work without hints. Done means those selected conversions are supported while ambiguous conversions still require type hints.

Written by the indexing model from the issue text.