Bulk gcore cleanup, replace core and alloc with std (#2735)

* gcore: replace `core` and `alloc` paths with `std`

* node-graph: remove unnecessary path prefix

* gcore: remove most `#[cfg(target_arch = "spirv")]`, keep some potentially useful ones

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Co-authored-by: Keavon Chambers <keavon@keavon.com>

node-graph/gcore/src/value.rs

@@ -1,6 +1,6 @@
 use crate::Node;
-use core::cell::{Cell, RefCell, RefMut};
-use core::marker::PhantomData;
+use std::cell::{Cell, RefCell, RefMut};
+use std::marker::PhantomData;
 
 #[derive(Default, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
 pub struct IntNode<const N: u32>;