add mrbc -h option

add comments for clear_all_old()

the old comment "clean all the black object as old" looks confusing,
it looks like "transform black object to old object", but indeed black
is old,

[mrbtest] Actual/Expect Order - ArgumentError and Array

Fix assignment of block as actual value and not expected

Remove pop that was done when not a return val

I replace 0 with NULL because struct pointer be should set NULL in.

I replace 'Pre-Conditions' with 'Preconditions' because the former is a ...