Update various parts of the text.

There's no such thing as "early ReferenceError" anymore (for things like 0++); they're all just "early SyntaxError" now. tc39/ecma262#691

Not sure whether it's a bug or wrong usage of Underscores.jl, but following example fails

v = [(x = rand(), y = rand()) for _ in 1:10]
@_ v |> sum(_.x, __) / length(__)

ERROR: MethodError: no method matching /(::var"#56#58"{Array{NamedTuple{(:x, :y),Tuple{Float64,Float64}},1}}, ::Int64)
Closest candidates are:
  /(::Missing, ::Number) at missing.jl:115
  /(::BigInt, ::Union{Int

Discuss the external API and the internal messaging passing API. Briefly explain the go loop as the engine of state maintenance and analog to closures.