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/src/from_xml_doc.md

@@ -588,7 +588,6 @@ a field, for consistency.
 #### Example without codec
 
 ```rust
-# extern crate alloc;
 # use xso::FromXml;
 #[derive(FromXml, Debug, PartialEq)]
 #[xml(namespace = "urn:example", name = "foo")]
@@ -606,7 +605,6 @@ assert_eq!(foo, Foo {
 #### Example with codec
 
 ```rust
-# extern crate alloc;
 # use xso::FromXml;
 #[derive(FromXml, Debug, PartialEq)]
 #[xml(namespace = "urn:example", name = "foo")]