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/fold.rs

@@ -859,8 +859,8 @@ mod test {
 
     // this version doesn't care about getting comments or docstrings in.
     fn fake_print_crate(s: &mut pprust::State,
-                        crate: &ast::Crate) -> io::IoResult<()> {
-        pprust::print_mod(s, &crate.module, crate.attrs)
+                        krate: &ast::Crate) -> io::IoResult<()> {
+        pprust::print_mod(s, &krate.module, krate.attrs)
     }
 
     // change every identifier to "zz"