Replace crate usage with krate

This patch replaces all `crate` usage with `krate` before introducing the
new keyword. This ensures that after introducing the keyword, there
won't be any compilation errors.

krate might not be the most expressive substitution for crate but it's a
very close abbreviation for it. `module` was already used in several
places already.

src/libsyntax/ext/base.rs

@@ -268,7 +268,7 @@ pub struct MacroCrate {
 }
 
 pub trait CrateLoader {
-    fn load_crate(&mut self, crate: &ast::ViewItem) -> MacroCrate;
+    fn load_crate(&mut self, krate: &ast::ViewItem) -> MacroCrate;
     fn get_exported_macros(&mut self, crate_num: ast::CrateNum) -> ~[~str];
     fn get_registrar_symbol(&mut self, crate_num: ast::CrateNum) -> Option<~str>;
 }