Use objectpath.Encoder to amortize the cost of encoding the paths of …

…multiple objects (#140)

We are using `objectpath.For` very heavily in our inference engine to
construct package-independent object identifiers. However, each
invocation comes with a cost of initializations (construction and
traversal of the scope tree).

Since x/tools v0.9.0 (we are currently on v0.11.0), there is an
`objectpath.Encoder` that "[amortizes the cost of encoding the paths of
multiple
objects](https://pkg.go.dev/golang.org/x/tools/go/types/objectpath#Encoder)".
This PR uses the encoder for better performance. There aren't/shouldn't
be any functionality changes.

A quick profile showing the timing before & after this PR for
`accumulation.run` function (the entrypoint function for our inference
engine):

before:

```
31.43s 22.49% |   go.uber.org/nilaway/accumulation.run
15.69s 49.92% |   go.uber.org/nilaway/inference.(*Engine).ObservePackage
8.11s 25.80%  |   go.uber.org/nilaway/inference.(*Engine).ObserveUpstream
3.33s 10.59%  |   go.uber.org/nilaway/inference.(*InferredMap).Export
...
```

after:

```
18.34s 14.63% |   go.uber.org/nilaway/accumulation.run
8.55s 46.62%  |   go.uber.org/nilaway/inference.(*Engine).ObserveUpstream
3.65s 19.90%  |   go.uber.org/nilaway/inference.(*InferredMap).Export
2.50s 13.63%  |   go.uber.org/nilaway/inference.(*Engine).ObservePackage
...
```

inference/primitive.go

@@ -125,6 +125,9 @@ type primitivizer struct {
 	// curDir is the current working directory, which is used to trim the prefix (e.g.,  random
 	// sandbox prefix if using bazel) from the file names for cross-package references.
 	curDir string
+	// objPathEncoder is used to encode object paths, which amortizes the cost of encoding the
+	// paths of multiple objects.
+	objPathEncoder *objectpath.Encoder
 }
 
 // newPrimitivizer returns a new and properly-initialized primitivizer.
@@ -172,6 +175,7 @@ func newPrimitivizer(pass *analysis.Pass) *primitivizer {
 		pass:                 pass,
 		upstreamObjPositions: upstreamObjPositions,
 		curDir:               cwd,
+		objPathEncoder:       &objectpath.Encoder{},
 	}
 }
 
@@ -193,7 +197,7 @@ func (p *primitivizer) fullTrigger(trigger annotation.FullTrigger) primitiveFull
 
 // site returns the primitive version of the annotation site.
 func (p *primitivizer) site(key annotation.Key, isDeep bool) primitiveSite {
-	objPath, err := objectpath.For(key.Object())
+	objPath, err := p.objPathEncoder.For(key.Object())
 	if err != nil {
 		// An error will occur when trying to get object path for unexported objects, in which case
 		// we simply assign an empty object path.