Introduce an "envelope" package

This patch introduces an "envelope" package which, for now, provides simple
utilities for getting the user and domain of an address.

It also changes the couriers to use it (but other implementations remain, will
be moved over in subsequent patches).

internal/envelope/envelope_test.go

@@ -0,0 +1,43 @@
+package envelope
+
+import (
+	"testing"
+
+	"blitiri.com.ar/go/chasquid/internal/set"
+)
+
+func TestSplit(t *testing.T) {
+	cases := []struct {
+		addr, user, domain string
+	}{
+		{"lalala@lelele", "lalala", "lelele"},
+	}
+
+	for _, c := range cases {
+		if user := UserOf(c.addr); user != c.user {
+			t.Errorf("%q: expected user %q, got %q", c.addr, c.user, user)
+		}
+		if domain := DomainOf(c.addr); domain != c.domain {
+			t.Errorf("%q: expected domain %q, got %q",
+				c.addr, c.domain, domain)
+		}
+	}
+}
+
+func TestDomainIn(t *testing.T) {
+	ls := set.NewString("domain1", "domain2")
+	cases := []struct {
+		addr string
+		in   bool
+	}{
+		{"u@domain1", true},
+		{"u@domain2", true},
+		{"u@domain3", false},
+		{"u", true},
+	}
+	for _, c := range cases {
+		if in := DomainIn(c.addr, ls); in != c.in {
+			t.Errorf("%q: expected %v, got %v", c.addr, c.in, in)
+		}
+	}
+}