Add specs() accessor to the PlatformEval type. (#423)

This helps `guppy` clients which don't want to `eval` the condition, and
instead want to decompose and analyze the condition. An example of one
such client can be found in:

* https://crrev.com/c/6259145/9/tools/crates/gnrt/lib/deps.rs#406
* https://crrev.com/c/6259145/9/tools/crates/gnrt/lib/gn.rs#513

guppy/src/platform/platform_eval.rs

@@ -159,7 +159,7 @@ pub struct PlatformEval<'g> {
 
 assert_covariant!(PlatformEval);
 
-impl PlatformEval<'_> {
+impl<'g> PlatformEval<'g> {
     /// Runs this evaluator against the given platform.
     pub fn eval(&self, platform: &Platform) -> EnabledTernary {
         let mut res = EnabledTernary::Disabled;
@@ -173,6 +173,11 @@ impl PlatformEval<'_> {
         }
         res
     }
+
+    /// Returns the target specs.
+    pub fn specs<'a>(&'a self) -> impl Iterator<Item = &'g TargetSpec> + 'a {
+        self.specs.iter()
+    }
 }
 
 #[derive(Clone, Debug)]