Update Kemal to 1.6.0 and remove Kilt

Kilt is unmaintained and the ECR templating logic has been
natively integrated into Kemal with the issues previously seen
having been resolved.

This commit is mostly a precursor to support the next Kemal
release which will add the ability to create error handlers for
raised exceptions.

See https://github.com/kemalcr/kemal/pull/688

src/ext/kemal_content_for.cr

@@ -1,16 +0,0 @@
-# Overrides for Kemal's `content_for` macro in order to keep using
-# kilt as it was before Kemal v1.1.1 (Kemal PR #618).
-
-require "kemal"
-require "kilt"
-
-macro content_for(key, file = __FILE__)
-  %proc = ->() {
-    __kilt_io__ = IO::Memory.new
-    {{ yield }}
-    __kilt_io__.to_s
-  }
-
-  CONTENT_FOR_BLOCKS[{{key}}] = Tuple.new {{file}}, %proc
-  nil
-end

src/ext/kemal_static_file_handler.cr

@@ -71,7 +71,7 @@ def send_file(env : HTTP::Server::Context, file_path : String, data : Slice(UInt
   filesize = data.bytesize
   attachment(env, filename, disposition)
 
-  Kemal.config.static_headers.try(&.call(env.response, file_path, filestat))
+  Kemal.config.static_headers.try(&.call(env, file_path, filestat))
 
   file = IO::Memory.new(data)
   if env.request.method == "GET" && env.request.headers.has_key?("Range")