Performance improvements for the event query API (#7319)

Rework the implementation of event query parsing and execution to
improve performance and reduce memory usage.

Previous memory and CPU profiles of the pubsub service showed query
processing as a significant hotspot. While we don't have evidence that
this is visibly hurting users, fixing it is fairly easy and self-contained.

Updates #6439.

Typical benchmark results comparing the original implementation (PEG) with the reworked implementation (Custom):
```
TEST                        TIME/OP  BYTES/OP  ALLOCS/OP  SPEEDUP   MEM SAVING
BenchmarkParsePEG-12       51716 ns  526832    27
BenchmarkParseCustom-12     2167 ns    4616    17         23.8x     99.1%
BenchmarkMatchPEG-12        3086 ns    1097    22
BenchmarkMatchCustom-12    294.2 ns      64     3         10.5x     94.1%
```

Components:
* Add a basic parsing benchmark.
* Move the original query implementation to a subdirectory.
* Add lexical scanner for Query expressions.
* Add a parser for Query expressions.
* Implement query compiler.
* Add test cases based on OpenAPI examples.
* Add MustCompile to replace the original MustParse, and update usage.

.github/workflows/coverage.yml

@@ -88,7 +88,7 @@ jobs:
           go-version: "1.17"
       - name: test & coverage report creation
         run: |
-          cat pkgs.txt.part.${{ matrix.part }} | xargs go test -mod=readonly -timeout 8m -race -coverprofile=${{ matrix.part }}profile.out
+          cat pkgs.txt.part.${{ matrix.part }} | xargs go test -mod=readonly -timeout 15m -race -coverprofile=${{ matrix.part }}profile.out
         if: env.GIT_DIFF
       - uses: actions/upload-artifact@v2
         with: