bitcoin: Compile time assert on index size

Currently we enforce that our code only runs on machines with a
certain pointer width (32 or 64 by failing to compile if pointer size
width is 16). One of the underlying reasons is because of requirements
in consensus code in Bitcoin Core which requires containers with more
than 2^16 (65536) items [0].

We can better express our requirements by asserting on Rust's index
size (the `usize` type).

As a side benefit, there is active work [1] to make Rust support
architectures where pointer width != idex size. With this patch applied
`rust-bitcoin` will function correctly even if that work progresses.

- [0] https://github.com/rust-bitcoin/rust-bitcoin/pull/2929#discussion_r1659399813
- [1] https://github.com/rust-lang/rust/issues/65473

bitcoin/src/lib.rs

@@ -37,12 +37,19 @@
 #![allow(clippy::manual_range_contains)] // More readable than clippy's format.
 #![allow(clippy::needless_borrows_for_generic_args)] // https://github.com/rust-lang/rust-clippy/issues/12454
 
-// Disable 16-bit support at least for now as we can't guarantee it yet.
-#[cfg(target_pointer_width = "16")]
-compile_error!(
-    "rust-bitcoin currently only supports architectures with pointers wider than 16 bits, let us
-    know if you want 16-bit support. Note that we do NOT guarantee that we will implement it!"
-);
+use core::mem;
+
+// We only support machines with index size of 4 bytes or more.
+//
+// Bitcoin consensus code relies on being able to have containers with more than 65536 (2^16)
+// entries in them so we cannot support consensus logic on machines that only have 16-bit memory
+// addresses.
+//
+// We specifically do not use `target_pointer_width` because of the possibility that pointer width
+// does not equal index size.
+//
+// ref: https://github.com/rust-bitcoin/rust-bitcoin/pull/2929#discussion_r1661848565
+internals::const_assert!(mem::size_of::<usize>() >= 4);
 
 #[cfg(bench)]
 extern crate test;