Improve the performance more

I improved the performance of parsing an HTTP request even more.

* Reduce calling `babel:octets-to-string`
  * cl-irregsexp can be used with octet vectors
* Add type declarations to commonly used functions like `get-header-block` and `convert-headers-plist`.
* Reduce `subseq` in `get-header-block` for memory efficiency.

#### Before (master branch)
```
(http-parse-test::RUN-benchmarks)
BENCH: Response Parse speed: 39.313s
BENCH: Request Parse speed: 0.596s
```

#### After (fast branch)
```
(http-parse-test::RUN-benchmarks)
BENCH: Response Parse speed: 39.561s
BENCH: Request Parse speed: 0.39s
```

In this benchmark, **34% faster** than before.