Add more functions that tell about operators, their associativity and how they parse

[dalum]

Ref: https://discourse.julialang.org/t/list-operator-associativity-in-the-documentation/5080

As far as I can tell, the rule of associativity is pretty simple and directly tied to the precedence. This, however, is not at all obvious. Hopefully this PR makes it more transparent to the user.

[JeffBezanson]

Nice. +, ++, and * are also non-associative (in the parser at least):

julia> dump(parse("a+b+c"))
Expr
  head: Symbol call
  args: Array{Any}((4,))
    1: Symbol +
    2: Symbol a
    3: Symbol b
    4: Symbol c
  typ: Any

[dalum]

I thought it was quite confusing that the precedence of unary operators were also not listed in the table, and found a few threads here and there suggesting that this is something people get confused about. Not sure if they read the manual, but now it's there. I also exposed the unary-op? lisp function as Base.isunaryoperator and added some docstrings for it and Base.isoperator.

I'm not sure if it makes sense to talk about associativity of unary operators, but the parser translates it into something right-associative, e. g.,

julia> :(√√√2)
:(√(√(√2)))

and not :(((√√)√)2), which would be the case for a left-associative (postfix) unary operator.

[StefanKarpinski]

Nice work on this PR!