More unsigned signed warning fixes

Refactor mrb_define_class() method in class.c.c

Add Object#instance_exec

Refactor mrb_ary_equal() func and mrb_ary_eql() func in array.c.

Add Range#eql? test case.

Refactor mrb_realloc() func in gc.c.

Unsigned signed warning fixes

Add mrb_class_get_under()

Fix a few unsigned signed comparison warnings

Fix Range#==

Refactor range_eql() func in rangec.c and add Range#eql? method test.

Refactor mrb_range_eq() func in range.c.

Refactor mrb_obj_eq() func in object.c.

add nil.to_a, nil.to_f, nil.to_i methods

refactor mrb_realloc() in gc.c

crc.c - replace POSIX with C99 header

Refactor mrb_hash_empty_p() func in hash.c

ArgumentError raised when mrb_get_args(mrb, "z", ...) receives substring.

Fix include path of ArduinoDue example