Runtime builtins -- implement sleep (and related functionality)

[cipriancraciun]

Tasks

• [x] implement (pause <time> [<randomize>]);
• [ ] implement (pause-retry <retry> [<pause-slice>] [<pause-slice-max>] [<pause-exponent-base>] [<randomize>]);
• [ ] implement (retry <thunk> [<max-retries>] [<pause-slice>] [<pause-slice-max>] [<pause-exponent-base>] [<randomize>])

Links

• https://en.wikipedia.org/wiki/Exponential_backoff
• https://cloud.google.com/storage/docs/exponential-backoff
• https://developers.google.com/api-client-library/java/google-http-java-client/backoff
• https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/