Marked reserve function as unsafe

[kitcatier]

https://github.com/koute/stdweb/blob/9b418d98df6fafaa4d4b87b04c304d0220292055/src/webcore/global_arena.rs#L69-L76 hello, if a function's entire body is unsafe, the function is itself unsafe and should be marked appropriately. Marking them unsafe also means that callers must make sure they know what they're doing.