add core.checkedint
GC: remove redundant null checks.
Merge pull request #839 from WalterBright/checkedint

add core.checkedint
add reference
Merge pull request #846 from WalterBright/add-ref

add reference
Merge pull request #845 from Safety0ff/removeuseless

GC: remove redundant null checks.
gc.realloc: If old block attributes are preserved, communicate them from reallocNoSync to realloc.