Use const array for unrolled Huffman table

Motivation

We've spent a long time with an unrolled Huffman-table to optimize the
Huffman decoding of HPACK headers. This works well, but because of some
limitations in Swift it's been impossible to embed this structure in the
binary directly: we've always had to alloc-and-copy on startup.
Additionally, due to type checker performance limitations, we've been
Base64 decoding it on startup, which isn't great either.

The introduction of const allows us to embed the knowledge of the fact
that this structure is compile-time constant in to the binary, improving
performance.

Modifications

Revert to the old layout, constify, and remove anything that blocks
that.

Result

Hopefully better performance and less dirty memory.

Author: Lukasa