Support embedding static resources into source files at compile time

[vekatze]

Embedding Static Files

This PR adds support for embedding static files into source files at compile time.

The compiler triggers recompilation when necessary by comparing modification times of static files and source files.

3-Step Usage

(1) Create a static file some-file.txt in a module:

Hello!

(2) Choose an alias of the static file and declare it in module.ens:

{
  // ..
  static {
    // (alias) == some-file
    some-file "rel/path/from/module/root/to/some-file.txt"
  }
  // ..
}

(3) Use static files from source files:

import {
  this.foo.bar,
  // ..
  static {some-file}, // syntax: static {ALIAS-1, ..., ALIAS-n}
}

define use-static-file(): unit {
  let t: &text = include-text(some-file) in // syntax: include-text(ALIAS)
  print(t) // => "Hello!"
}