Add support for at-nospecialize

[ararslan]

I started working on this but haven't quite nailed it down yet. Here's my start if anyone else wants to have a go:

diff --git a/src/Compat.jl b/src/Compat.jl
index c0c82d4..3007fab 100644
--- a/src/Compat.jl
+++ b/src/Compat.jl
@@ -662,6 +662,27 @@ module Sys
     end
 end

+if VERSION < v"0.7.0-DEV.969"
+    # If any type annotations are provided, we'll just ignore them for now
+    macro nospecialize(ex)
+        if isa(ex, Symbol)                                    # @nospecialize(x)
+            Expr(:(::), quot(ex), :ANY)
+        elseif ex.head == :(=) && isa(ex.args[1], Symbol)     # @nospecialize(x=1)
+            Expr(:(=), Expr(:(::), quot(ex.args[1]), :ANY), quot(ex.args[2]))
+        elseif ex.head == :(=) && ex.args[1].head == :(::)    # @nospecialize(x::T=1)
+            Expr(:(=), Expr(:(::), quot(ex.args[1].args[1]), :ANY), quot(ex.args[1].args[2]))
+        elseif ex.head == :(...) && isa(ex.args[1], Symbol)   # @nospecialize(x...)
+            Expr(:(...), Expr(:(::), quot(ex.args[1]), :ANY))
+        elseif ex.head == :(...) && ex.args[1].head == :(::)  # @nospecialize(x::T...)
+            Expr(:(...), Expr(:(::), quot(ex.args[1].args[1]), :ANY))
+        else
+            error("improper use of @nospecialize")
+        end
+    end
+else
+    import Base: @nospecialize
+end
+
 include("deprecated.jl")

 end # module Compat

It basically just takes the @nospecialize annotation for a function argument and turns it into an ::ANY annotation on the argument.

The quoting here isn't working quite right, since, for example, the returned expression from @nospecialize(x) is actually :x::Main.ANY as the above stands. And we can't use esc here instead, since that produces Main.x::Main.ANY, which is wrong in the context of specifying a function signature.