Override read_exact and write_all

[Kixunil]

Some types override these methods to provide optimizations. Using
default implementations would bypass those overrides and lead to worse
performance.

Overriding the methods to delegate implementations directly solves the
problem.

[cuviper]

Do you have examples of types that override these specific methods?

If we do these, should we also override/forward as many other io methods as we can? There are still some left, even constrained by our MSRV, like read_to_string and write_fmt and some BufRead stuff.

[Kixunil]

&[u8] is a typical example and I do it for my own type too.

should we also override/forward as many other io methods as we can?

Probably yes, I think it's OK to skip MSRV-restricted.

[cuviper]

Thanks! I added those few more.