Prewalk the syntax tree in at-static evaluation

Post-walking the syntax tree meant the `if-else` chains were evaluated
in opposite order from how they are normally processed, and this
has consequences for things like checks on the existence of symbols:

```julia
@static if VERSION >= v"1.1" && (Sys.isfreebsd() || Sys.isdragonfly())
    ...
    @static if Sys.freebsd()
        ...
    end
end
```

Under normal evaluation rules, this is valid — we have previously
checked that the version is great enough for the `Sys.isfreebsd()` and
`Sys.isdragonfly()` functions to exist, so the inner case doesn't need
to check the version again.

With the post-walk evaluation order, though, the inner `if` is
evaluated first, and this threw errors as we tried to actually run
the conditional. By switching to a pre-walk order, the outer `if` will
kill the block of code on pre-v1.1 versions of Julia, and the inner
case will never be evaluated.

Author: jmert

src/mmap.jl

@@ -1,5 +1,4 @@
 using BitFlags
-using MacroTools: @capture, flatten, isexpr, postwalk
 
 import Base: INVALID_OS_HANDLE
 import Mmap
@@ -104,7 +103,7 @@ Base.cconvert(::Type{T}, mf::MmapFlags) where {T <: Integer} = T(mf)
         MADV_AUTOSYNC = 7
         MADV_NOCORE = 8
         MADV_CORE = 9
-        @static if VERSION >= v"1.1" && Sys.isfreebsd()
+        @static if Sys.isfreebsd()
             MADV_PROTECT = 10
         else
             MADV_INVAL = 10

src/staticexpand.jl

@@ -1,3 +1,5 @@
+using MacroTools: flatten, isexpr, prewalk, postwalk
+
 """
     @staticexpand @a_macro begin
         # some code...
@@ -16,7 +18,7 @@ macro staticexpand(expr::Expr)
     sentinel = gensym("sentinel")
     isexpr(expr, :macrocall) || error("Expected a macro to expand")
     # Evaluate the static statements
-    expr′ = postwalk(expr) do ex
+    expr′ = prewalk(expr) do ex
         # Search for @static macro
         isexpr(ex, :macrocall) || return ex
         ex = ex::Expr