Implicit Values: context; Fix lvalue selector assignments; Fix offset_of* for using fields.

core/mem.odin

@@ -131,7 +131,7 @@ init_arena_from_memory :: proc(using a: ^Arena, data: []byte) {
 }
 
 init_arena_from_context :: proc(using a: ^Arena, size: int) {
-	backing = current_context().allocator
+	backing = context.allocator
 	memory = new_slice(u8, 0, size)
 	temp_count = 0
 }