Remove unused String::create_reserved
Use dirty object style in String::from_bytearray
Remove unneeded OnStack in VariableScope

The original problem was not due to the variable not being marked
OnStack, but because of another initialization ordering issue. This was
added during debugging, but should have been removed before committing
the code.
Ensure that the saved TZ is actually 0 terminated

Valgrind caught this by marking it as an uninitialized jump, because
strcmp was used later on.