implement RFCs 147 and 155, etc.

This commit:

- Implements RFC 147: "all functions must be safe"

- Implements RFC 155: "explicit Context parameter"

- Implements the pending breaking change #141: reject assign syntax in `init`
  (which was used to initialize late resources)

- Refactors code generation to make it more readable -- there are no more random
  identifiers in the output -- and align it with the book description of RTFM
  internals.

- Makes the framework hard depend on `core::mem::MaybeUninit` and thus will
  require nightly until that API is stabilized.

- Fixes a ceiling analysis bug where the priority of the system timer was not
  considered in the analysis.

- Shrinks the size of all the internal queues by turning `AtomicUsize` indices
  into `AtomicU8`s.

- Removes the integration with `owned_singleton`.

macros/src/lib.rs

@@ -288,9 +288,9 @@ mod syntax;
 pub fn app(args: TokenStream, input: TokenStream) -> TokenStream {
     // Parse
     let args = parse_macro_input!(args as syntax::AppArgs);
-    let items = parse_macro_input!(input as syntax::Input).items;
+    let input = parse_macro_input!(input as syntax::Input);
 
-    let app = match syntax::App::parse(items, args) {
+    let app = match syntax::App::parse(input.items, args) {
         Err(e) => return e.to_compile_error().into(),
         Ok(app) => app,
     };
@@ -304,5 +304,5 @@ pub fn app(args: TokenStream, input: TokenStream) -> TokenStream {
     let analysis = analyze::app(&app);
 
     // Code generation
-    codegen::app(&app, &analysis).into()
+    codegen::app(&input.ident, &app, &analysis).into()
 }