Log Message:
Updated package to latest version, 1.2.1. Incorporated license change
(BSD3 to ISC) that was not mentioned in changelog. Changes include:

- Allow signals to be created and `S.stop`ped instantaneously (#18)
  Previously this could lead to failed assertions in updates (e.g.
  `S.bind` trying to switch to an uninitialized signal). Thanks
  to Arthur Wendling for the report.
- Fix implementation of `S.Bool.flip`, its initial value on creation
  could be wrong in dynamic creation (#17). Thanks to Arthur Wendling
  for the report.
- Fix bug in `S.Option.value` with `` `Always`` on `S.const None` (#19).
  Thanks to Arthur Wendling for the report.