program-entrypoint: Add a BumpAllocator constructor (#284)

* program-entrypoint: Add a BumpAllocator constructor

#### Problem

The current version of program-entrypoint doesn't work because the
allocator can't be instantiated in the macros. The start and len fields
are private, and so can't be written to in the macro, which exists in
the scope of the program.

#### Summary of changes

Add a new constructor that can be used by the macro.

* Make function const, call from unsafe block

* Rename

Author: joncinque

program-entrypoint/src/lib.rs

@@ -221,9 +221,11 @@ macro_rules! custom_heap_default {
     () => {
         #[cfg(all(not(feature = "custom-heap"), target_os = "solana"))]
         #[global_allocator]
-        static A: $crate::BumpAllocator = $crate::BumpAllocator {
-            start: $crate::HEAP_START_ADDRESS as usize,
-            len: $crate::HEAP_LENGTH,
+        static A: $crate::BumpAllocator = unsafe {
+            $crate::BumpAllocator::with_fixed_address_range(
+                $crate::HEAP_START_ADDRESS as usize,
+                $crate::HEAP_LENGTH,
+            )
         };
     };
 }
@@ -333,6 +335,19 @@ impl BumpAllocator {
             len: arena.len(),
         }
     }
+
+    /// Creates the allocator tied to specific range of addresses.
+    ///
+    /// # Safety
+    /// This is unsafe in most situations, unless you are totally sure that the
+    /// provided start address and length can be written to by the allocator,
+    /// and that the memory will be usable for the lifespan of the allocator.
+    ///
+    /// For Solana on-chain programs, a certain address range is reserved, so
+    /// the allocator can be given those addresses.
+    pub const unsafe fn with_fixed_address_range(start: usize, len: usize) -> Self {
+        Self { start, len }
+    }
 }
 
 /// Integer arithmetic in this global allocator implementation is safe when