shrink mp4::Segment 128 -> 112 bytes (on 64-bit)

* don't store sizes of mp4-format sample indexes; recalculate them.
   * keep SampleIndexIterator position as a u32 rather than a usize.

This is 960 bytes for a 60-minute mp4; another small cache usage improvement.

src/main.rs

@@ -41,6 +41,7 @@ extern crate fnv;
 extern crate http_entity;
 extern crate hyper;
 #[macro_use] extern crate lazy_static;
+extern crate lazycell;
 extern crate libc;
 #[macro_use] extern crate log;
 extern crate lru_cache;