fix #7188

This makes displaying and parsing of symbols more symmetric, as discussed in #7188. I'm not sure why this was a 0.3 issue, but here's code to make it work anyways.


I implemented symbol"" (`@symbol_str`), for normal usage (wrapped in a QuoteNode).


I also implemented a macro version `Meta.@symbol("")` for getting back a unadorned Symbol, so that even things like anonymous functions would be a bit more directly parseable:

```julia
julia> macro test()
         quote
             function x() end
             function y() x() end
         end
       end

julia> @test
#124#y (generic function with 1 method)

julia> code_lowered(ans,())
1-element Array{Any,1}:
 :($(Expr(:lambda, {}, {{},{},{}}, :(begin  # none, line 6:
        return @symbol("#123#x")()
    end))))
```
Are you sure removing `LUTridiagonal` from exporting is related to this PR? Seems like a commit mix-up.