Change the IV_EXPECT macro into something useful

[JeffGarland]

Please see this PR #75

for a discussion of the IV_EXPECT macro. It appears that the intent of this macro was to conditionally do pre-condition checks using asserts when enabled. This is in effect c++26 contracts. Also now in the standard there's a group of Hardened preconditions that use contracts that apply to in-place vector. It would be great to try out the experimental compilers with contracts against these checks. For non-experimental compilers these precondition checks can simply be asserts in the function body -- with an option to compile them out.