Make it possible to use flip-link

flip-link (https://github.com/knurling-rs/flip-link) moves the stack to the bottom of the ram,
so that we can reliably detect when running out of stack space.

As per knurling-rs/flip-link#62 this requires using alloc-cortex-m differently,
using a static allocation (in .bss)

examples/printerdemo/rust/main.rs

@@ -65,9 +65,7 @@ pub fn main() {
 #[cfg(feature = "mcu-pico-st7789")]
 #[sixtyfps_rendering_backend_mcu::entry]
 fn main() -> ! {
-    sixtyfps_rendering_backend_mcu::init_board(sixtyfps_rendering_backend_mcu::BoardConfig {
-        heap_size: 120 * 1024,
-    });
+    sixtyfps_rendering_backend_mcu::init_board();
     printerdemo_main();
     loop {}
 }

sixtyfps_runtime/rendering_backends/mcu/pico_st7789.rs

@@ -18,14 +18,13 @@ fn oom(_: core::alloc::Layout) -> ! {
 }
 use alloc_cortex_m::CortexMHeap;
 
+const HEAP_SIZE: usize = 128 * 1024;
+static mut HEAP: [u8; HEAP_SIZE] = [0; HEAP_SIZE];
+
 #[global_allocator]
 static ALLOCATOR: CortexMHeap = CortexMHeap::empty();
 
-pub struct BoardConfig {
-    pub heap_size: usize,
-}
-
-pub fn init_board(config: BoardConfig) {
+pub fn init_board() {
     let mut pac = pac::Peripherals::take().unwrap();
     let core = pac::CorePeripherals::take().unwrap();
 
@@ -78,8 +77,7 @@ pub fn init_board(config: BoardConfig) {
         bl.set_high().unwrap();
     }
 
-    let start = cortex_m_rt::heap_start() as usize;
-    unsafe { ALLOCATOR.init(start, config.heap_size) }
+    unsafe { ALLOCATOR.init(&mut HEAP as *const u8 as usize, core::mem::size_of_val(&HEAP)) }
 
     display.init(&mut delay).unwrap();
     display.set_orientation(st7789::Orientation::Landscape).unwrap();