Macro arguments are treated as globals

It seems that macro arguments are treated as globals (see `@global x`)

```julia
julia> solve(:(macro f(x) end))
:(macro (@global f)(@global x)
#= REPL[12]:1 =#
end)
```

Compare this to a function:

```julia
julia> solve(:(function f(x) end))
[]function (@global f)(@x)
#= REPL[13]:1 =#
end
```

Also, `macro (@global f)` is a bit hard to use. Would `@global var"@f"` make sense?