There is a hot path through `opt_normalize_projection_type`:
- `try_start` does a cache lookup (#1).
- The result is a `NormalizedTy`.
- There are no unresolved type vars, so we call `complete`.
- `complete` does *another* cache lookup (rust-lang#2), then calls
  `SnapshotMap::insert`.
- `insert` does *another* cache lookup (rust-lang#3), inserting the same value
  that's already in the cache.

This patch optimizes this hot path by introducing `complete_normalized`,
for use when the value is known in advance to be a `NormalizedTy`. It
always avoids lookup rust-lang#2. Furthermore, if the `NormalizedTy`'s
obligations are empty (the common case), we know that lookup rust-lang#3 would be
a no-op, so we avoid it, while inserting a Noop into the `SnapshotMap`'s
undo log.

This patch changes `opt_normalize_project_type` so it appends
obligations to a given obligations vector, instead of returning a new
obligations vector.

This change avoids lots of allocations. In the most extreme case, for a
clean "Check" build of serde it reduces the total number of allocations
by 20%.