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

test/cases/eval.zig

@@ -251,3 +251,15 @@ fn comptimeIterateOverFnPtrList() {
     assert(performFn('o', 0) == 1);
     assert(performFn('w', 99) == 99);
 }
+
+fn evalSetDebugSafetyAtCompileTime() {
+    @setFnTest(this);
+
+    const result = comptime fnWithSetDebugSafety();
+    assert(result == 1234);
+}
+
+fn fnWithSetDebugSafety() -> i32{
+    @setDebugSafety(this, true);
+    return 1234;
+}