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/math.zig

@@ -1,3 +1,5 @@
+const assert = @import("debug.zig").assert;
+
 pub const Cmp = enum {
     Equal,
     Greater,
@@ -66,3 +68,18 @@ fn getReturnTypeForAbs(comptime T: type) -> type {
     }
 }
 
+fn testMath() {
+    @setFnTest(this);
+
+    assert(%%mulOverflow(i32, 3, 4) == 12);
+    assert(%%addOverflow(i32, 3, 4) == 7);
+    assert(%%subOverflow(i32, 3, 4) == -1);
+    assert(%%shlOverflow(i32, 0b11, 4) == 0b110000);
+
+    comptime {
+        assert(%%mulOverflow(i32, 3, 4) == 12);
+        assert(%%addOverflow(i32, 3, 4) == 7);
+        assert(%%subOverflow(i32, 3, 4) == -1);
+        assert(%%shlOverflow(i32, 0b11, 4) == 0b110000);
+    }
+}