Drop AutoHashEquals dependency, other minor updates

[christopher-dG]

I don't precisely remember why we needed AutoHashEquals (I think maybe Julia v0.x didn't implement hash, which Dict keys require, for new structs), but one test did still depend on it. Regardless, it's not needed anymore.

Other changes:

• Make struct fields concrete (mostly AbstractString -> String)
• Replace some big interpolated expressions with the varargs form of print

[omus]

If your fields of your custom types don't use concrete types then you may need to change how hashing works. On Julia 1.0.3:

julia> using AutoHashEquals

julia> struct Foo1
           x::AbstractString
       end

julia> hash(Foo1("A")) == hash(Foo1(SubString("A")))
false

julia> @auto_hash_equals struct Foo2
           x::AbstractString
       end

julia> hash(Foo2("A")) == hash(Foo2(SubString("A")))
true

julia> struct Foo3
           x::String
       end

julia> hash(Foo3("A")) == hash(Foo3(SubString("A")))
true

[christopher-dG]

Maybe I'm missing something, but I think that that's ultimately okay, since the keys of Template.plugins are the plugin types rather than values.
Which would imply that AutoHashEquals was never actually required... I might have just gotten the idea that I needed when I wrote tests along the lines of Set(values(t.plugins)) == Set([...]).

[omus]

I think you're safe to remove this now. I do think it's only safe to be removed because you also switched AbstractString -> String

[omus]

I think I prefer the string interpolation but this is fine too

[christopher-dG]

I find it a lot easier to edit the parts of the string when a) it's properly syntax highlighted and b) there are less parens to deal with