Optimise wrappers

[Victorystick]

Reduce the overhead of the CommonJS wrappers using a helper function __commonjs_wrapper.

function __commonjs_wrapper(fn, module) {
  return module = { exports: {} }, fn(module, module.exports), module.exports;
}

Result after minification

// before ~75 bytes
var a=function(o){o.exports;return o.exports=r.Foo,o.exports}({exports:{}})

// after ~35 bytes
var a = r(function(r){r.exports=e.Foo})

// webpack (for comparison) ~35 bytes
function(a,b,c){a.exports=c(5).Foo}

See #12 (comment)

[Rich-Harris]

Closing this in favour of #20, which has the same result but doesn't include all the unmergeable stuff from other PRs