Move the force_eval! macro into the math module

This fixes the cross-inclusion into the compiler builtins

src/lib.rs

@@ -12,12 +12,6 @@
 #![deny(warnings)]
 #![no_std]
 
-macro_rules! force_eval {
-    ($e:expr) => {
-        unsafe { ::core::ptr::read_volatile(&$e); }
-    }
-}
-
 mod math;
 
 #[cfg(todo)]

src/math/mod.rs

@@ -1,3 +1,9 @@
+macro_rules! force_eval {
+    ($e:expr) => {
+        unsafe { ::core::ptr::read_volatile(&$e); }
+    }
+}
+
 mod fabs;
 mod fabsf;
 mod fmodf;