Add MemoryView.export and MemoryView#release

[kou]

fix #79

Users can release memory views explicitly before process exit.

Reported by xtkoba. Thanks!!!

[mrkn]

I think these two methods are useful. But, it seems less clear that get with a block implicitly releases the memory view at the end of the block. I want to change the name of get if we can find a more appropriate name.

[kou]

get is borrowed from rb_memory_view_get_func_t.

Other candidates:

• MemoryView.open: like File.open but MemoryView doesn't open anything
• MemoryView.acquire: release and acquire pair is often used in "lock" context but the current context isn't "lock"
• MemoryView.map
• MemoryView.view

[kou]

We choose MemoryView.export.