globset: make GlobSet::new public

For users of globset who already have a `Vec<Glob>` (or similar),
the current API requires them to iterate over their `Vec<Glob>`,
calling `GlobSetBuilder::add` for each `Glob`, thus constructing a new
`Vec<Glob>` internal to the GlobSetBuilder. This makes the consuming
code unnecessarily verbose. (There is unlikely to be any meaningful
performance impact of this, however, since the cost of allocating a new
`Vec` is likely marginal compared to the cost of glob compilation.)

Instead of taking a `&[Glob]`, we accept an iterator of anything that
can be borrowed as a `&Glob`. This required some light refactoring of
the constructor, but nothing onerous.

Closes #3066

crates/globset/src/glob.rs

@@ -80,6 +80,12 @@ pub struct Glob {
     tokens: Tokens,
 }
 
+impl AsRef<Glob> for Glob {
+    fn as_ref(&self) -> &Glob {
+        self
+    }
+}
+
 impl PartialEq for Glob {
     fn eq(&self, other: &Glob) -> bool {
         self.glob == other.glob && self.opts == other.opts