Text codecs allow to customize the conversion of data from/to XML,
in particular in two scenarios:
1. When the type for which the behaviour is to be defined comes from a
foreign crate, preventing the implementation of
FromXmlText/IntoXmlText.
2. When there is not one obvious, or more than one sensible, way to
convert a value to XML text and back.
Previously, we only enforced the existence of at most one `#[xml(text)]`
field only at code generation time for `FromXml`. This change enforces
it at parsing time, which is more consistent and allows for a clearer
error message.
This is bare-bones and is missing many features which we intend to add
in future commits, such as parsing from attributes whose names differ
from the field names and parsing into non-String types.
