fix memcpy-param-overlap
temporary macros for checked arithmetic functions
add function for checked mrb_int addition
add function for checked mrb_int subtraction
Implement day of week methods.
Merge pull request #2199 from cremno/use-memmove

fix memcpy-param-overlap
Merge pull request #2201 from take-cheeze/day_of_week

Implement day of week methods.
Merge pull request #2200 from cremno/add-functions-for-safe-addition-and-subtraction

Add functions for safe addition and subtraction