Proposal: Consider BufferReader and BufferWriter implementation in std.mem

[arkadiuszwojcik]

I'm quite new to the Zig community, so I may not have seen a lot, but recently, while submitting a PR to the MicroZig project, I had to implement BufferReader and BufferWriter types. This is quite common pattern (I think) and Zig also use it in one place where it is called FixedBufferReader. I think something like that is very useful and should be promoted to std.mem namespace, although FixedBufferReader itself have some specyfic functions (like: readUleb128) I would not expect in generic implementation. FixedBufferWriter would be useful in cases when we have to join together a lot os small byte chunks while serializing. So together FixedBufferReader and FixedBufferWriter would be very nice addition to std.mem namespace in my opinion.