corrections in cpp promotion
added make_safe_literal
separated integers from other types
dropped example91
updated CMakeLists.txt and Jamfile.v2 accordingly
adjust rest of program accordingly
this should
a) factor casts out of checked operations and put them in safe_base_operations
b) simplify interval which will not explicitly check for overflows etc.
First milestone - make test_add_native pass all addition tests.
Turns out that these changes should also better position us to handle other underlying types - but we're not there yet.
Added example for composition with other types - example boost rational and boost multi precision
Added performance test using boost multi precision
Many other small corrections
