add simple rust example

examples/rust/build.sh

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+rustc --target=wasm32-unknown-unknown --crate-type cdylib -C opt-level="z" -C "link-args=--import-memory --initial-memory=262144 -zstack-size=65536" -o tunnel.wasm tunnel.rs && \
+wasm-opt -Oz -o tunnel.wasm tunnel.wasm

examples/rust/readme.txt

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+A small example how to produce somewhat reasonably small MicroW8
+carts in rust.
+
+A nightly rust compiler is needed for the unstable sqrtf32
+intrinsic.
+
+Simply compiling with rustc as shown in build.sh results in a
+339 byte tunnel.wasm. Using wasm-opt this can be reduced to
+244 bytes.
+
+When you disassemble this wasm file using wasm2wat you can see
+these globals and exports:
+
+(global (;0;) i32 (i32.const 65536))
+(global (;1;) i32 (i32.const 65536))
+(export "__data_end" (global 0))
+(export "__heap_base" (global 1))
+
+They are meant to be used for heap allocations and stack for any
+values that are not simple scalars (i32, f32, etc.). Since our
+code doesn't actually use any of that, we can just delete them
+in a text editor and assemble the code again with wat2wasm.
+
+This gives us a 200 byte wasm file. Running this through
+uw8-tool pack brings us to the final size of 137 bytes.

examples/rust/tunnel.rs

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+#![no_std]
+#![feature(core_intrinsics)]
+
+#[link(wasm_import_module = "math")]
+extern "C" {
+    fn atan2(x: f32, y: f32) -> f32;
+}
+
+fn sqrt(v: f32) -> f32 {
+    unsafe { core::intrinsics::sqrtf32(v) }
+}
+
+#[no_mangle]
+pub fn tic(time: i32) {
+    unsafe {
+        for i in 0..320 * 256 {
+            let t = time as f32 / 10 as f32;
+            let x = (i % 320 - 160) as f32;
+            let y = (i / 320 - 128) as f32;
+            let d = 20000 as f32 / sqrt(x * x + y * y + 1 as f32);
+            let u = atan2(x, y) * 512f32 / 3.141;
+            let c = ((d + t).to_int_unchecked::<i32>() ^ (u + t).to_int_unchecked::<i32>()) as u8;
+            *((120 + i) as *mut u8) = c;
+        }
+    }
+}
+
+#[panic_handler]
+fn panic(_: &core::panic::PanicInfo) -> ! {
+    loop {}
+}