Update API

[leobalter]

I had a sync meeting with @littledan and @caridy and we defined some changes to the current API. I plan to change the repo's readme and polyfill accordingly:

On:

• importBinding: async, does not need to evaluate strings or CSP relaxing. Refs #8 and #4
• eval wrapping callable returns: _sync, string evaluation, needs CSP relaxing+
• implicit function wrapping: auto wrapping still creates a frozen bridge function that won't leak other values Ref #6

Off:

• Function: wrapping functions from importBinding and eval eliminates the need for returning functions. Ref #6
• AsyncFunction
• import. Ref #4
• wrappedFunctionCallback Ref #6