Version 2.0.0, with breaking changes.

- Changed error handling -- VCL failure for unrecoverable errors,
  see the ERRORS section in the manual (README). This is a breaking
  change.
- set .compile() is unnecessary and deprecated, but legal for the
  time being.
- Add set .subroutine() and .check_call()
- Requires VRT 13.0, compatible with Varnish 6.6