Box the BrotliState value because it is very large

[wesleywiser]

This reduces the size of BrotliDecoder from 2592 bytes to 8 bytes.

Fixes #172

[Nemo157]

bors r+

[bors]

Build succeeded:

• cargo clippy
• cargo deny bans licenses sources
• cargo doc --cfg docsrs
• cargo fmt --check
• cargo hack check --all-targets --feature-powerset
• cargo hack check --feature-powerset
• cargo test
• cargo test --shallow-minimal-versions
• cargo test (stable, i686-pc-windows-msvc, windows-latest)
• cargo test (stable, i686-unknown-linux-gnu, ubuntu-latest)
• cargo test (stable, x86_64-apple-darwin, macos-latest)
• cargo test (stable, x86_64-pc-windows-msvc, windows-latest)
• cargo test (stable, x86_64-unknown-linux-gnu, ubuntu-latest)