Change in `let` expressions for julia v0.7/master breaks match expressions

[jmert]

I'm relatively confident that the PR JuliaLang/julia#23553 is the reason the following example breaks when trying to use julia compiled from master:

julia> using MacroTools; using MacroTools: @q
...
julia> @match :(function tryme(x); x; end) begin
         function f_(args__) body_ end => @q $f($args...) = $body
       end
ERROR: UndefVarError: f not defined

I've put together a patch to the internal function bindinglet which seems to fix the issue — I could probably work up a pull request for it. (I'm mostly a github lurker, so rather unfamiliar with the process...)

[jmert]

Fixed by #57.