This allows stateful or configurable codecs without having to express
all configuration in the type name itself. For example, we could have a
Base64 type with configurable Base64 engines without having to duplicate
the Base64 type itself.
(Note that the different engines in the Base64 crate are values, not
types.)
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.
