[object] Reiterate on DicomObject

- replace `to_dicom_value` with `to_primitive_value`
   - simplifies implementations
   and requires consumers to
   depend on other methods
   when working with sequences
- remove method `meta`
   - treat meta information attributes like any other attribute,
   retrievable through the same methods
- [core] add either crate
- impl many DICOM traits to `either::Either`
- implement DicomObject for FileMetaTable
- reimplement DICOM traits for FileDicomObject
  so that users can retrieve
  either meta info or main data set info

core/Cargo.toml

@@ -11,6 +11,7 @@ readme = "README.md"
 
 [dependencies]
 chrono = { version = "0.4.31", default-features = false, features = ["std", "clock"] }
+either = "1.13.0"
 itertools = "0.12"
 num-traits = "0.2.12"
 safe-transmute = "0.11.0"

core/src/value/mod.rs

@@ -23,6 +23,8 @@ pub use self::primitive::{
     ValueType,
 };
 
+pub use either::Either;
+
 /// An aggregation of one or more elements in a value.
 pub type C<T> = SmallVec<[T; 2]>;
 
@@ -43,6 +45,39 @@ pub trait DicomValueType: HasLength {
     fn cardinality(&self) -> usize;
 }
 
+impl<L, R> HasLength for Either<L, R>
+where
+    L: HasLength,
+    R: HasLength,
+{
+    fn length(&self) -> Length {
+        match self {
+            Either::Left(l) => l.length(),
+            Either::Right(r) => r.length(),
+        }
+    }
+}
+
+impl<L, R> DicomValueType for Either<L, R>
+where
+    L: DicomValueType,
+    R: DicomValueType,
+{
+    fn value_type(&self) -> ValueType {
+        match self {
+            Either::Left(l) => l.value_type(),
+            Either::Right(r) => r.value_type(),
+        }
+    }
+
+    fn cardinality(&self) -> usize {
+        match self {
+            Either::Left(l) => l.cardinality(),
+            Either::Right(r) => r.cardinality(),
+        }
+    }
+}
+
 /// Representation of a full DICOM value, which may be either primitive or
 /// another DICOM object.
 ///