Turn allocate_value_lazy into a static "constructor"

This changes allocate_value_lazy to be a static "constructor" of
struct value.

I considered trying to change value to use ordinary new/delete, but it
seems to me that due to reference counting, we may someday want to
change these static constructors to return value_ref_ptr instead.

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