header: introduce uv_loop_size()

We add a function called uv_loop_size which returns the size of
the loop. It is an additional function which complements
`uv_handle_size` and` uv_req_size` in order to provide full FFI
support.

Signed-off-by: Fedor Indutny <fedor@indutny.com>

include/uv.h

@@ -288,6 +288,11 @@ UV_EXTERN uv_loop_t* uv_loop_new(void);
  */
 UV_EXTERN void uv_loop_delete(uv_loop_t*);
 
+/*
+ * Returns size of the loop struct, useful for dynamic lookup with FFI
+ */
+UV_EXTERN size_t uv_loop_size(void);
+
 /*
  * This function runs the event loop. It will act differently depending on the
  * specified mode:

src/uv-common.c

@@ -67,6 +67,11 @@ size_t uv_req_size(uv_req_type type) {
 #undef XX
 
 
+size_t uv_loop_size(void) {
+  return sizeof(uv_loop_t);
+}
+
+
 uv_buf_t uv_buf_init(char* base, unsigned int len) {
   uv_buf_t buf;
   buf.base = base;