Add inline attributes to Vec's MutBuf methods (#80)

I found this significantly improved a
[benchmark](https://gist.github.com/danburkert/34a7d6680d97bc86dca7f396eb8d0abf)
which calls `bytes_mut`, writes 1 byte, and advances the pointer with
`advance_mut` in a pretty tight loop. In particular, it seems to be the
inline annotation on `bytes_mut` which had the most effect. I also took
the opportunity to simplify the bounds checking in advance_mut.

before:

```
test encode_varint_small  ... bench:         540 ns/iter (+/- 85) = 1481 MB/s
```

after:

```
test encode_varint_small  ... bench:         422 ns/iter (+/- 24) = 1895 MB/s
```

As you can see, the variance is also significantly improved.

Interestingly, I tried to change the last statement in `bytes_mut` from

```
&mut slice::from_raw_parts_mut(ptr, cap)[len..]
```

to

```
slice::from_raw_parts_mut(ptr.offset(len as isize), cap - len)
```

but, this caused a very measurable perf regression (almost completely
negating the gains from marking bytes_mut inline).

src/buf/buf_mut.rs

@@ -699,23 +699,25 @@ impl<T: AsMut<[u8]> + AsRef<[u8]>> BufMut for io::Cursor<T> {
 }
 
 impl BufMut for Vec<u8> {
+    #[inline]
     fn remaining_mut(&self) -> usize {
         usize::MAX - self.len()
     }
 
+    #[inline]
     unsafe fn advance_mut(&mut self, cnt: usize) {
-        let cap = self.capacity();
-        let len = self.len().checked_add(cnt)
-            .expect("overflow");
-
-        if len > cap {
-            // Reserve additional
-            self.reserve(cap - len);
+        let len = self.len();
+        let remaining = self.capacity() - len;
+        if cnt > remaining {
+            // Reserve additional capacity, and ensure that the total length
+            // will not overflow usize.
+            self.reserve(cnt - remaining);
         }
 
-        self.set_len(len);
+        self.set_len(len + cnt);
     }
 
+    #[inline]
     unsafe fn bytes_mut(&mut self) -> &mut [u8] {
         use std::slice;