Enable building with older Rust versions

Prevent timeouts on large differences

Improved conflicts highlighting + highlighting TNG

With this release, many more conflicts are recognized, and changes vs
non-changed line parts are marked much more clearly.

Also, this release introduces a `--unchanged-style=experimental`
command line option. Try `export RIFF=--unchanged-style=experimental`
and report back!

Tune conflicts highlighting

Both by clearly highlighting the markers, and by improving how we color
the conflicting areas themselves.

And remember: "git config --global merge.conflictstyle diff3" is your
friend!

Further improve what we highlight and not

With this release, we go from plain LCS to the Patience Diff algorithm.
"Patience" is marketed as producing more human friendly diffs.

Additionally, post processing has been tuned to take better decisions on
which differences should be highlighted and not.

Tune what gets highlighted and not

Tune leading whitespace formatting

We should always have lowlighted leading whitespace for both old and new
lines, but that was forgotten about.

Don't highlight leading whitespace

Tune file names highlighting

Before this release, file names were all white.

With this release, the path has normal intensity, but the actual name
of the file is still bold.

Improved conflicts highlighting

Especially for diff3 conflicts, where we now show unchanged lines vs
base in context color.

With this release, we are also better at detecting conflict markers in
`git diff` output.