Add embassy-cortex-m crate.

- Move Interrupt and InterruptExecutor from `embassy` to `embassy-cortex-m`.
- Move Unborrow from `embassy` to `embassy-hal-common` (nothing in `embassy` requires it anymore)
- Move PeripheralMutex from `embassy-hal-common` to `embassy-cortex-m`.

embassy-macros/src/macros/cortex_m_interrupt.rs

@@ -0,0 +1,66 @@
+use darling::FromMeta;
+use proc_macro2::TokenStream;
+use quote::quote;
+use std::iter;
+use syn::ReturnType;
+use syn::{Type, Visibility};
+
+use crate::util::ctxt::Ctxt;
+
+#[derive(Debug, FromMeta)]
+struct Args {}
+
+pub fn run(args: syn::AttributeArgs, mut f: syn::ItemFn) -> Result<TokenStream, TokenStream> {
+    let _args = Args::from_list(&args).map_err(|e| e.write_errors())?;
+
+    let ident = f.sig.ident.clone();
+    let ident_s = ident.to_string();
+
+    // XXX should we blacklist other attributes?
+
+    let valid_signature = f.sig.constness.is_none()
+        && f.vis == Visibility::Inherited
+        && f.sig.abi.is_none()
+        && f.sig.inputs.is_empty()
+        && f.sig.generics.params.is_empty()
+        && f.sig.generics.where_clause.is_none()
+        && f.sig.variadic.is_none()
+        && match f.sig.output {
+            ReturnType::Default => true,
+            ReturnType::Type(_, ref ty) => match **ty {
+                Type::Tuple(ref tuple) => tuple.elems.is_empty(),
+                Type::Never(..) => true,
+                _ => false,
+            },
+        };
+
+    let ctxt = Ctxt::new();
+
+    if !valid_signature {
+        ctxt.error_spanned_by(
+            &f.sig,
+            "`#[interrupt]` handlers must have signature `[unsafe] fn() [-> !]`",
+        );
+    }
+
+    ctxt.check()?;
+
+    f.block.stmts = iter::once(
+        syn::parse2(quote! {{
+            // Check that this interrupt actually exists
+            let __irq_exists_check: interrupt::#ident;
+        }})
+        .unwrap(),
+    )
+    .chain(f.block.stmts)
+    .collect();
+
+    let result = quote!(
+        #[doc(hidden)]
+        #[export_name = #ident_s]
+        #[allow(non_snake_case)]
+        #f
+    );
+
+    Ok(result)
+}