Runtime builtins -- implement "unique tokens" for build-time, run-time and boot-time

[cipriancraciun]

Tasks

• add runtime variables for:
  • build-time token; (i.e. the same for the same build, but different for each build;)
  • run-time token; (i.e. the same for the same execution, but different for each execution;)
  • boot-time token; (i.e. on Linux use /proc/sys/kernel/random/boot_id;)
• expose them as builtins;