Add tests for coverage.

Undefine class method new rather than instance method.

Applies to NilClass, TrueClass, FalseClass, Fixnum, and Float. Add
mrb_undef_class_method for DRYness.

fix segmentation fault in Array#last

Use memcpy instead of strncpy

fix: SIZE_MAX is not defined in "stdint.h"

add test cases in test/t/float.rb

Remove initialization of mt.

Modify allocation size of mrb->irep.

mrb_load_* should return value

Remove magic numbers in mrb_funcall()

Modify proc_lambda

Modify return value of 'respond_to?'

Undef Class#extend_object.