Attempt to retain current location when loading a different platform

For each target platform, if the current location exists within that
platform then generate a link directly to it. Otherwise link to the root
of the crate in that platform like normal.

Author: Nemo157

src/test/fakes.rs

@@ -97,10 +97,16 @@ impl<'a> FakeRelease<'a> {
     }
 
     pub(crate) fn default_target(mut self, target: &'a str) -> Self {
+        self = self.add_target(target);
         self.default_target = Some(target);
         self
     }
 
+    pub(crate) fn add_target(mut self, target: &str) -> Self {
+        self.doc_targets.push(target.into());
+        self
+    }
+
     pub(crate) fn binary(mut self, bin: bool) -> Self {
         self.has_docs = !bin;
         if bin {

src/web/crate_details.rs

@@ -42,7 +42,7 @@ pub struct CrateDetails {
     github_issues: Option<i32>,
     pub(crate) metadata: MetaData,
     is_library: bool,
-    doc_targets: Option<Json>,
+    pub(crate) doc_targets: Option<Json>,
     license: Option<String>,
     documentation_url: Option<String>,
 }