Add value::set_modifiable

This introduces a value::set_modifiable and changes a couple of spots
to use it.

I'm not completely sure the comments by deprecated_modifiable are
correct any more.  Perhaps they should be removed and the method
renamed.  Like so many before me, though, I've deferred investigation
of the issue.

Approved-By: Simon Marchi <simon.marchi@efficios.com>