Simplify currentUser syntax

[orodio]

fcl.currentUser being a function confuses people, they are expecting it to be an instance of a class or object.

We should be able to simplify the syntax so that the following is true:

import {currentUser} from "@onflow/fcl"

// before
currentUser().subscribe(callback)
// after
currentUser.subscribe(callback)

// before
currentUser().snapshot()
// after
currentUser.snapshot()

// before
currentUser().authenticate
// after
currentUser.authenticate

// before
currentUser().authorization
// after
currentUser.authorization

// before
currentUser().asArg
// after
currentUser.asArg

[chasefleming]

Addressed in #796