Implement Windows support

[sunshowers]

Still TODO:

• Get cargo-compare running on Windows.

Some of the failures we're seeing are:

thread 'tests::workspace_tests::cargo_guppy::proptest_compare' panicked at 'compute_diff succeeded: failed to read `\\?\D:\a\cargo-guppy\cargo-guppy\internal-tools/benchmarks\Cargo.toml`

Caused by:
    The filename, directory name, or volume label syntax is incorrect. (os error 123)', internal-tools\cargo-compare\src\tests\proptest_helpers.rs:42:10

thread 'tests::workspace_tests::inside_outside::proptest_compare' panicked at 'compute_diff succeeded: package collision in the lockfile: packages external v0.1.0 (D:\a\cargo-guppy\cargo-guppy\fixtures\workspace\inside-outside\external) and external v0.1.0 (D:\a\cargo-guppy\cargo-guppy\fixtures\workspace\inside-outside\workspace\internal-macro\../../external) are different, but only one can be written to lockfile unambiguously', internal-tools\cargo-compare\src\tests\proptest_helpers.rs:42:10

[sunshowers]

Tagging this as "help wanted" -- I don't really use or target Windows for development so it would be good to have experienced Windows developers fix up any remaining issues.

[sunshowers]

We can probably address this by switching out cargo-compare to use the new cargo tree support for printing out resolved features.