Upgrade `constrain` to `2.0.0-alpha.1`.
Upgrade `constrain` to `2.0.0-alpha.1`.

Constrain now includes an iterator which we'll be using with the logic
for sorting out the LevelDB specific arguments pulled into this module.

https://github.com/bigeasy/constrain/releases/tag/v2.0.0-alpha.1

Closes #253.

bigeasy/locket,bigeasy/locket