Both ways do the same thing but I think the compiler might have an
easier time optimizing `appendNTimes` because it does less
things/the path is shorter.

I have not done any benchmarking at runtime but have compared the
instruction count of both ways a little here: https://zig.godbolt.org/z/vr193W9oj
`b` (`appendNTimes`) is ~103 instructions while `a`
(`writer().writeByteNTimes`) is ~117 instructions.

And looking at the implementation of `writeByteNTimes`, it only seems to
buffer up 256 bytes before doing another `writeAll` which for
`std.ArrayList` probably means another allocation, whereas when directly
using `appendNTimes`, the entire exact additional capacity required is known from the start.

Either way, this would be more consistent anyway.