tough: cache function should not take Option<GENERIC>

[webern]

Accepting an Option<T> as a function parameter is bad form because it means callers must specify a type (any type) when passing None.

tough/tough/src/cache.rs

Line 23 in ac9a347

targets_subset: Option<&[S]>,

The function should be changed from

    pub fn cache<P1, P2, S>(
        &self,
        metadata_outdir: P1,
        targets_outdir: P2,
        targets_subset: Option<&[S]>,
        cache_root_chain: bool,
    ) -> Result<()> {}

To either this:

    pub fn cache<P1, P2, S>(
        &self,
        metadata_outdir: P1,
        targets_outdir: P2,
        targets_subset: &[S],
        cache_root_chain: bool,
    ) -> Result<()> {}

Where is_empty means the same thing as is_none.

... or this

    pub fn cache<P1, P2, S>(
        &self,
        metadata_outdir: P1,
        targets_outdir: P2,
        targets_subset: Option<&[&str]>,
        cache_root_chain: bool,
    ) -> Result<()> {}

If is_empty and None need to carry different meanings.