Rename IndexVec::last → last_index

As I've been trying to replace a `Vec` with an `IndexVec`, having `last` exist on both but returning very different types makes the transition a bit awkward -- the errors are later, where you get things like "there's no `ty` method on `mir::Field`" rather than a more localized error like "hey, there's no `last` on `IndexVec`".

So I propose renaming `last` to `last_index` to help distinguish `Vec::last`, which returns an element, and `IndexVec::last_index`, which returns an index.

(Similarly, `Iterator::last` also returns an element, not an index.)

compiler/rustc_trait_selection/src/solve/search_graph/mod.rs

@@ -54,7 +54,7 @@ impl<'tcx> SearchGraph<'tcx> {
     /// Whether we're currently in a cycle. This should only be used
     /// for debug assertions.
     pub(super) fn in_cycle(&self) -> bool {
-        if let Some(stack_depth) = self.stack.last() {
+        if let Some(stack_depth) = self.stack.last_index() {
             // Either the current goal on the stack is the root of a cycle...
             if self.stack[stack_depth].has_been_used {
                 return true;