auto merge of #11803 : sfackler/rust/simple-mac, r=brson

Now that procedural macros can be implemented outside of the compiler,
it's more important to have a reasonable API to work with. Here are the
basic changes:

* Rename SyntaxExpanderTTTrait to MacroExpander, SyntaxExpanderTT to
    BasicMacroExpander, etc. I think "procedural macro" is the right
    term for these now, right? The other option would be SynExtExpander
    or something like that.

* Stop passing the SyntaxContext to extensions. This was only ever used
    by macro_rules, which doesn't even use it anymore. I can't think of
    a context in which an external extension would need it, and removal
    allows the API to be significantly simpler - no more
    SyntaxExpanderTTItemExpanderWithoutContext wrappers to worry about.

src/test/auxiliary/macro_crate_test.rs

@@ -27,8 +27,8 @@ macro_rules! unexported_macro (() => (3))
 #[macro_registrar]
 pub fn macro_registrar(register: |Name, SyntaxExtension|) {
     register(token::intern("make_a_1"),
-        NormalTT(~SyntaxExpanderTT {
-            expander: SyntaxExpanderTTExpanderWithoutContext(expand_make_a_1),
+        NormalTT(~BasicMacroExpander {
+            expander: expand_make_a_1,
             span: None,
         },
         None));