allow handlers to be named 'main'

`#[init]`, `#[idle]` and `#[task]` handlers can now be named `main`

fixes #311

examples/t-htask-main.rs

@@ -0,0 +1,20 @@
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+use cortex_m_semihosting::debug;
+use panic_semihosting as _;
+
+#[rtfm::app(device = lm3s6965)]
+const APP: () = {
+    #[init]
+    fn init(_: init::Context) {
+        rtfm::pend(lm3s6965::Interrupt::UART0)
+    }
+
+    #[task(binds = UART0)]
+    fn main(_: main::Context) {
+        debug::exit(debug::EXIT_SUCCESS);
+    }
+};

examples/t-idle-main.rs

@@ -0,0 +1,20 @@
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+use cortex_m_semihosting::debug;
+use panic_semihosting as _;
+
+#[rtfm::app(device = lm3s6965)]
+const APP: () = {
+    #[init]
+    fn init(_: init::Context) {
+    }
+
+    #[idle]
+    fn main(_: main::Context) -> ! {
+        debug::exit(debug::EXIT_SUCCESS);
+        loop {}
+    }
+};

examples/t-init-main.rs

@@ -0,0 +1,15 @@
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+use cortex_m_semihosting::debug;
+use panic_semihosting as _;
+
+#[rtfm::app(device = lm3s6965)]
+const APP: () = {
+    #[init]
+    fn main(_: main::Context) {
+        debug::exit(debug::EXIT_SUCCESS);
+    }
+};

examples/t-stask-main.rs

@@ -0,0 +1,24 @@
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+use cortex_m_semihosting::debug;
+use panic_semihosting as _;
+
+#[rtfm::app(device = lm3s6965)]
+const APP: () = {
+    #[init(spawn = [main])]
+    fn init(cx: init::Context) {
+        cx.spawn.main().ok();
+    }
+
+    #[task]
+    fn main(_: main::Context) {
+        debug::exit(debug::EXIT_SUCCESS);
+    }
+
+    extern "C" {
+        fn UART0();
+    }
+};