zstd: surface implicit default level 3 via libzstd constant

[hdhoang]

instead of c-like flag 0.

this implements #156, i hope it's helpful

[Nemo157]

Thanks!

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)