encode: tighten max encoded length estimation

[mina86]

Instead of using 8/5 ratio (i.e. 1.6) when estimating the length of the encoded output use 1.5 which is closer to the actual 1.37 and produces simpler expression.

[Nemo157]

bors r+

[bors[bot]]

Merge conflict.

[Nemo157]

bors retry

[bors[bot]]

Build succeeded:

• cargo audit
• cargo clippy
• cargo doc --cfg docsrs
• cargo fmt --check
• 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)