Fix overload ambiguity errors

[adayton1]

care::host_device_ptr had two constructors that sometimes led to compilation errors due to ambiguous overload resolution.

CARE_HOST_DEVICE host_device_ptr(std::nullptr_t from);
CARE_HOST_DEVICE explicit host_device_ptr(size_t elems);

Compilation would fail when constructing a host_device_ptr from the integer literal 0 because the compiler can't decide which overload to pick. Additionally, nvcc erroneously reports ambiguity when constructing from an int in certain cases.

Using a constraint resolves the ambiguity in both of those situations. The nullptr constructor will now only accept nullptr or a variable of type std::nullptr_t. The size_t constructor will get all the rest, including 0.