Reduce boilerplate + avoid warning when overriding context for the current scope

[eyalroz]

There are several dozen places in our code where need to set a context for the scope (typically, the function body). We do so by defining a local CADRe variable :

cuda::context::current::detail_::scoped_override_t set_context_for_this_scope(expr);

with expr being some expression yielding a context handle. This is fine, but - it would not be a bad idea to mark this variable as const (various IDEs are all for it). Also, it's a lot of typing. Let's use a macro, then:

#define CAW_SET_SCOPE_CONTEXT(context_handle_expr_) \
const cuda::context::current::detail_::scoped_override_t caw_context_for_this_scope_(context_handle_expr_)

this might look a little ugly (WHO LIKES ALL CAPS?), but it conveys the intention even more clearly when reading the code, and it's 24 characters + the length of expr, vs. 78 characters + the length of expr, which is not just less typing, but less cognitive load when reading the code.