7386: Handle box with allocator r=flodiebold a=tweksteen
In 1.49.0, the definition of Box was modified to support an optional
Allocator[1]. Adapt the parsing of the `box` keyword to supply the
expected number of parameters to the constructor.
[1] f288cd2e17Fixes: #6956
Co-authored-by: Thiébaud Weksteen <tweek@google.com>