Avoid memcpy() on copying structures.

Fix wrong storage of data when hex-style string is loaded

This patch fixes corruption of data when hex-style string is
loaded.

I saw following phenomenon in an ubuntu/amd64 environment.

Test code:
str = "\x0\x1\x2\x03\x04\x05\x06\a\b\t\n\v\f\r\x0E\x0F\x10"
p str[-1] == "\0x10"

Output(binary program with mrbc -B):
false

Output(script with mruby):
true

Fix disappearance of a local variable when return statement is called

Fix degrades by #490. (Some changes are reverted.)

Commit of #18dd60c1 causes disappearance of a local variable
if return statement is called without arguments.

This patch fixes incorrect value of stack pointer.

test program:
def test_return_cond
  return if nil
  obj = 123
  p obj
  p obj.class
end
test_return_cond

output(commit #18dd60c1):
main
Object

output(includes this patch):
123
Fixnum

Avoid memcpy() on copying structure.

Add more strict warning checks