Add introducers

- add the `Introducer` types
- get introducers from the certificate store (and log them)

src/types/introducer.rs

@@ -0,0 +1,33 @@
+//
+// husk-milter
+//
+// introducers
+//
+
+use sequoia_wot as wot;
+use wot::{CertSynopsis, Certification};
+use sequoia_openpgp::{policy::StandardPolicy, Cert, KeyID};
+
+#[derive(Debug, Clone)]
+pub struct Introducer {
+    pub cert: CertSynopsis,
+    pub certification: Certification,
+}
+
+impl Introducer {
+
+    pub fn can_introduce(&self, email: &str) -> bool {
+        if let Some(regexset) = self.certification.regular_expressions() {
+            if regexset.matches_everything() {
+                // return false, because unlimited certifications are 
+                // ignored.
+                false
+            } else {
+                regexset.is_match(email)
+            }
+        } else {
+            false
+        }
+    }
+}
+