[API] Go code to implement the Arvados API permissions model
A function that takes an Arvados user U, an Arvados read/write/manage permission P, and an arbitrary Arvados object O. It returns true if U has P permission to O, and false otherwise.
This is a measurable start to replacing some API services, especially Websockets, in Go.
TBD: More fully specify the interface, and any others necessary.