RUSTSEC-2020-0095: difference is unmaintained

[github-actions]

difference is unmaintained

Details
Status	unmaintained
Package	difference
Version	2.0.0
URL	johannhof/difference.rs#45
Date	2020-12-20

The author of the difference crate is unresponsive.

Maintained alternatives:

• dissimilar

• similar

• treediff

• diffus

See advisory page for additional details.

[jyn514]

Upstream bug:

$ cargo tree -i difference
difference v2.0.0
└── mockito v0.30.0
    [dev-dependencies]
    └── docs-rs v0.6.0 (/home/joshua/src/rust/docs.rs)

[jyn514]

lipanski/mockito#132

[syphar]

upstream issue is resolved