Conversions between types

Associate conversions with the most specific type involved. [FIXME: needs RFC]

When in doubt, prefer to_`to_/`/as_`as_/`/into_`into_to` to from_`from_`, because they are more ergonomic to use (and can be chained with other methods).

For many conversions between two types, one of the types is clearly more "specific": it provides some additional invariant or interpretation that is not present in the other type. For example, str`stris more specific than&[u8]`, since it is a utf-8 encoded sequence of bytes.

Conversions should live with the more specific of the involved types. Thus, str`strprovides both theas_bytesmethod and thefrom_utf8constructor for converting to and from&[u8]values. Besides being intuitive, this convention avoids polluting concrete types like&[u8]` with endless conversion methods.

Explicitly mark lossy conversions, or do not label them as conversions. [FIXME: needs RFC]

If a function's name implies that it is a conversion (prefix from_`from_,`, as_`as_,`, to_`to_or` or into_`into_), but the function loses information, add a suffix_lossy` or otherwise indicate the lossyness. Consider avoiding the conversion name prefix.