Instead of reading a byte at a time in a loop we copy the relevant bytes into
a temporary vector of size eight. We can then read the value from the temporary
vector using a single u64 read. LLVM seems to be able to optimize this
almost scarily good.
This is just an unnecessary trait that no one's ever going to parameterize over
and it's more useful to just define the methods directly on the types
themselves. The implementors of this type almost always don't want
inner_mut_ref() but they're forced to define it as well.
- `Buffer.lines()` returns `LineIterator` which yields line using
`.read_line()`.
- `Reader.bytes()` now takes `&mut self` instead of `self`.
- `Reader.read_until()` swallows `EndOfFile`. This also affects
`.read_line()`.
