`target()` as used in parsing "let" and "if let" implements parsing
nested tuples and structs. But it does not implement parsing literals.
The functions `match_variant()` and `with_parameters()` as used in
parsing "when" blocks do not implement parsing nested structs, but it
implements parsing literals.
This PR combines `match_variant()` and `with_parameters()` into
`target()`, so that all `{%when%}` support nested structs, too.
