Add dependency on MacroTools to avoid Base.remove_linenums!

[pdeffebach]

After #245, we have some Base.remove_linenums! scattered around. Initially I thought this was harmless, but it was not. See the following

julia> macro linenums_macro(arg)
           if arg isa Expr && arg.head == :block && length(arg.args) == 1 && arg.args[1] isa LineNumberNode
               esc(:(true))
           else
               esc(:(false))
           end
       end
@linenums_macro (macro with 1 method)

On master:

julia> df = DataFrame(a = [1], b = [2]);

julia> @transform(df, y = @linenums_macro begin end)
1×3 DataFrame
 Row │ a      b      y     
     │ Int64  Int64  Bool  
─────┼─────────────────────
   1 │     1      2  false

On this branch

julia> @transform(df, y = @linenums_macro begin end)
1×3 DataFrame
 Row │ a      b      y    
     │ Int64  Int64  Bool 
─────┼────────────────────
   1 │     1      2  true

Basically, macros inside @transform blocks might make assumptions about line number nodes, and we don't want to mess with that. I apologize for using Base.remove_linenums! earlier.

To implement this, I finally bit the bullet and added a dependency on MacroTools.jl. It's just not worth re-inventing the wheel anymore when it comes to these sorts of issues. Every other macro package today has MacroTools as a dependency, so I might do so as well.

We do remove line numbers still, but we only do it when unblock-ing an expression or working with :block-based expression, where as Base.remove_linenums! acts recursively.

I have added a testset so this should be good to merge.

[pdeffebach]

I should ping @nalimilan and @bkamins here to clarify that this is ready for a review.

[bkamins]

I assume all is OK. I am not an expert in MacroTools.jl, but I agree with the direction of changes.

[nalimilan]

Review challenge: only make suggestions on whitespace. :-D