Revert "Revert once SPL updates to Solana 1.8.1"

This reverts commit f5767bb.

Author: mvines

examples/rust/custom-heap/src/entrypoint.rs

@@ -5,13 +5,11 @@
 use solana_program::{
     account_info::AccountInfo,
     entrypoint,
-    entrypoint::{ProgramResult, HEAP_LENGTH},
+    entrypoint::{ProgramResult, HEAP_LENGTH, HEAP_START_ADDRESS},
     pubkey::Pubkey,
 };
 use std::{alloc::Layout, mem::size_of, ptr::null_mut, usize};
 
-const HEAP_START_ADDRESS: u64 = 0x300000000;
-
 /// Developers can implement their own heap by defining their own
 /// `#[global_allocator]`.  The following implements a dummy for test purposes
 /// but can be flushed out with whatever the developer sees fit.