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.
