Move Replaceable mixin param defaults to base template
Move Replaceable mixin param defaults to base template

Summary:
[Replaceable] Move `Replaceable` mixin param defaults to base template.

Saves a few lines overall.

Reviewed By: WillerZ

Differential Revision: D7421561

fbshipit-source-id: f56d1ffdd081b0a8a20cb7a492f06c9647445782

rklabs/folly,facebook/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly