put version and version operator in the same Option in the Atom struct

All atoms must either have a version with a version operator, or have
no version and no version operator. Putting these in the same Option
helps encode that into the type system.

src/atom/mod.rs

@@ -106,15 +106,23 @@ pub struct UseDep {
 #[derive(Clone, Debug, Get)]
 pub struct Atom {
     blocker: Option<Blocker>,
-    version_operator: Option<VersionOperator>,
     category: Category,
     name: Name,
-    version: Option<Version>,
+    version: Option<(VersionOperator, Version)>,
     slot: Option<Slot>,
     #[get(kind = "deref")]
     usedeps: Vec<UseDep>,
 }
 
+impl Atom {
+    pub fn version_operator(&self) -> Option<VersionOperator> {
+        match self.version {
+            Some((operator, _)) => Some(operator),
+            None => None,
+        }
+    }
+}
+
 impl fmt::Display for Blocker {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
@@ -299,7 +307,7 @@ impl fmt::Display for Atom {
             write!(f, "{blocker}")?;
         }
 
-        if let Some(version_operator) = self.version_operator.as_ref() {
+        if let Some(version_operator) = self.version_operator().as_ref() {
             write!(f, "{version_operator}")?;
         }
 
@@ -307,7 +315,7 @@ impl fmt::Display for Atom {
         write!(f, "/")?;
         write!(f, "{}", self.name)?;
 
-        if let Some(version) = self.version.as_ref() {
+        if let Some((_, version)) = self.version.as_ref() {
             write!(f, "-{version}")?;
         }