Implement basic in-memory session store

Sessions are stored in a non-persistent Hash.  Only String values are allowed.  A reaper fiber regularly removes any sessions which expire due to inactivity.

spec/session_spec.cr

@@ -0,0 +1,57 @@
+require "./spec_helper"
+
+describe "Session" do
+  it "can establish a session" do
+    sid = nil
+    existing = nil
+    get "/" do |env|
+      sess = env.session
+      existing = sess["token"]?
+      sid = sess.id
+      sess["token"] = "abc"
+      "Hello"
+    end
+
+    # make first request without any cookies/session
+    request = HTTP::Request.new("GET", "/")
+    response = call_request_on_app(request)
+    # dup headers due to Crystal#2920
+    headers = response.headers.dup
+
+    # verify we got a cookie and session ID
+    cookie = response.headers["Set-Cookie"]?
+    cookie.should_not be_nil
+    response.cookies[Kemal::Sessions::NAME].value.should eq(sid)
+    lastsid = sid
+    existing.should be_nil
+
+    # make second request with cookies to get session
+    request = HTTP::Request.new("GET", "/", headers)
+    response = call_request_on_app(request)
+
+    # verify we got cookies and we could see values set
+    # in the previous request
+    cookie2 = response.headers["Set-Cookie"]?
+    cookie2.should_not be_nil
+    cookie2.should eq(cookie)
+    response.cookies[Kemal::Sessions::NAME].value.should eq(lastsid)
+    existing.should eq("abc")
+  end
+
+  it "can prune old sessions" do
+    s = Kemal::Sessions::STORE
+    s.clear
+
+    Kemal::Sessions.prune!
+
+    id = "foo"
+    s[id] = Kemal::Sessions::Session.new(id)
+    s.size.should eq(1)
+    Kemal::Sessions.prune!
+    s.size.should eq(1)
+
+    s[id].last_access_at = (Time.now - 1.week).epoch_ms
+    Kemal::Sessions.prune!
+    s.size.should eq(0)
+  end
+end