lang: funcs: Add deploy package with readfile related functions

This adds a readfile function to actually access files from our deploy.
A fun side effect is that we can even access our own code! In general,
it's a good reminder that you should only run trusted code on your own
infrastructure. This also includes a fancy new test case.

lang/interpret_test/TestAstFunc2/deploy-readfile0/mod1/main.mcl

@@ -0,0 +1,6 @@
+import "deploy"
+
+# relative paths for us
+$f = "/files/file3" # real file is in: /mod1/files/file3
+$f3 = deploy.abspath($f)
+$x3 = deploy.readfile($f)