Release Splice version 3.0.0-alpha.39.
Release Splice version 3.0.0-alpha.39.

 * Upgrade `b-tree` to `2.0.0-alpha.79`.
 * Upgrade `turnstile` to `6.0.0-alpha.50`.
 * Upgrade `destructible` to `7.0.0-alpha.38`.
 * Upgrade `riffle` to `2.0.0-alpha.54`.

Closes #208.

bigeasy/splice,bigeasy/splice