Introduce set_lval method on value

This introduces the set_lval method on value, one step toward removing
deprecated_lval_hack.  Ultimately I think the goal should be for some
of these set_* methods to be replaced with constructors; but I haven't
done this, as the series is already too long.  Other 'deprecated'
methods can probably be handled the same way.

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