lots of miscellaneous things all in one big commit

* add `@compileLog(...)` builtin function
   - Helps debug code running at compile time
   - See #240
 * fix crash when there is an error on the start value of a slice
 * add implicit cast from int and float types to int and float
   literals if the value is known at compile time
 * make array concatenation work with slices in addition to
   arrays and c string literals
 * fix compile error message for something not having field access
 * fix crash when `@setDebugSafety()` was called from a
   function being evaluated at compile-time
 * fix compile-time evaluation of overflow math builtins.
 * avoid debug safety panic handler in builtin.o and compiler_rt.o
   since we use no debug safety in these modules anyway
 * add compiler_rt functions for division on ARM
   - Closes #254
 * move default panic handler to std.debug so users can
   call it manually
 * std.io.printf supports a width in the format specifier

std/debug.zig

@@ -13,6 +13,27 @@ pub fn assert(ok: bool) {
     if (!ok) @unreachable()
 }
 
+var panicking = false;
+/// This is the default panic implementation.
+pub coldcc fn panic(message: []const u8) -> unreachable {
+    // TODO
+    // if (@atomicRmw(AtomicOp.XChg, &panicking, true, AtomicOrder.SeqCst)) { }
+    if (panicking) {
+        // Panicked during a panic.
+        // TODO detect if a different thread caused the panic, because in that case
+        // we would want to return here instead of calling abort, so that the thread
+        // which first called panic can finish printing a stack trace.
+        os.abort();
+    } else {
+        panicking = true;
+    }
+
+    %%io.stderr.printf("{}\n", message);
+    %%printStackTrace();
+
+    os.abort();
+}
+
 pub fn printStackTrace() -> %void {
     %return writeStackTrace(&io.stderr);
     %return io.stderr.flush();