[Important] Double-Free Fix, Bit32 Compatibility

This release of sol2 addresses a double-free bug within the library present for closures and other things using the user abstraction. Users are encouraged to upgrade basically right now.

Fixes:

• #281 Latest bug where closure gets doubled freed and it seemed like a referencing issue
• Twitter bug reported by @eevee https://twitter.com/eevee/status/801255925440856065
• #282 bit32 compat when defining LUA_COMPAT_52 or LUA_COMPAT_BITLIB