mdspan: Add optional bounds checking

Both default_accessor and layout_{left, right, stride}::mapping::operator() have in-bounds access as a precondition. This permits implementations to add bounds checking, using whatever mechanism they like (as precondition violations are undefined behavior).

• Add a configure-time option to turn on bounds checking.
• Use macros to protect bounds checking and error reporting code, including header file includes like <iostream>. (The test and branch have a nonzero run-time cost. Also, code for printing error messages can costs compilation time and possibly also performance (e.g., extra registers in device code).)
• Add a test that only runs if bounds checking is turned on. Consider adding a build for that case.

Start by tracing default_accessor and layout_{left, right, stride}::mapping::operator() to understand their in-bounds preconditions. Define the configure-time switch and macro boundaries, including error-reporting includes, then add a test that runs only when bounds checking is enabled. Done means the option, guarded checks, and conditional test are integrated into the build.