Update html5ever to 0.31.0

[Kobzol]

The total build is faster, as there are ~10 less dependencies, while the leaf crate has ~10% more executed instructions.

time -p CARGO_INCREMENTAL=0 cargo check ; cargo clean
# 5.16
# 4.23

time -p CARGO_INCREMENTAL=0 cargo build ; cargo clean
# 5.5
# 4.4

time -p CARGO_INCREMENTAL=0 cargo build --release ; cargo clean
# 5.9
# 4.7

Related issue: #2024

[nnethercote]

Looks good except for the whitespace changes in README.md.