Revert "xso-proc: Replace std stuff with alloc/core stuff"

This reverts commit 4e5f0bc961.

Unfortunately, in std contexts, the `alloc` crate is not imported. That
means we cannot rely on it being accessible and it in fact breaks builds
of crates which are not `no_std`.

Fixes #155.

xso-proc/src/lib.rs

@@ -18,8 +18,6 @@ return to `xso` for more information**. The documentation of
 **You have been warned.**
 */
 
-extern crate alloc;
-
 // Wondering about RawTokenStream vs. TokenStream?
 // syn mostly works with proc_macro2, while the proc macros themselves use
 // proc_macro.