New import rules.

lib/std/builtin.c3

@@ -2,6 +2,7 @@
 // Use of this source code is governed by the MIT license
 // a copy of which can be found in the LICENSE_STDLIB file.
 module std::builtin;
+import libc;
 
 fault VarCastResult
 {

lib/std/mem_allocator_fn.c3

@@ -1,4 +1,5 @@
 module std::mem;
+import libc;
 
 private fn void*! null_allocator_fn(void *data, usize bytes, usize alignment, void* old_pointer, AllocationKind kind)
 {