Add a convenience function pb_get_encoded_size()

There is minimal size penalty from this, and it is probably much more
intuitive to use than PB_OSTREAM_SIZING itself.

This has been suggested before also, but I ended up refusing it back
them. Reconsidering it now, I see that an intuitive API is much better
than any amount of documentation explaining a non-intuitive API.

Update issue 16
Status: FixedInGit

tests/encode_unittests/encode_unittests.c

@@ -281,6 +281,15 @@ int main()
         TEST(WRITES(pb_encode_delimited(&s, IntegerContainer_fields, &msg),
                     "\x09\x0A\x07\x0A\x05\x01\x02\x03\x04\x05"))
     }
+
+    {
+        IntegerContainer msg = {{5, {1,2,3,4,5}}};
+        size_t size;
+        
+        COMMENT("Test pb_get_encoded_size.")
+        TEST(pb_get_encoded_size(&size, IntegerContainer_fields, &msg) &&
+             size == 9);
+    }
     
     {
         uint8_t buffer[10];