various improvements to parser

[JeffBezanson]

• move call to short-form-function-loc inside parse-assignment
  this is faster and fixes nested cases like f(x)=g(x)=1
• fix a case where an unexpected "foo" message was missing quotes
• optimize common case of arglists of comma-separated simple tokens
• some other small optimizations
• put code back in precedence order
• separate parse-Nary and parse-comma so they're easier to read
• cleaner code for parse-unary and parse-factor

release-0.6:

julia> open("bigarray.txt","w") do f
        show(f, rand(100000))
       end

julia> @time parse(readstring("bigarray.txt"));
  2.380243 seconds (100.02 k allocations: 3.244 MiB)

This PR:

julia> @time parse(readstring("bigarray.txt"));
  1.401921 seconds (100.02 k allocations: 3.244 MiB)

This only affects parsing speed of long, simple argument lists.