InBuffer作ってみました。
OutBufferをちょっといじった程度の物なので、これでいいならとっくに標準に入ってる気はしますが・・・。
何か指摘があればお願いします。

class InBuffer
{
ubyte data[];
uint offset;

invariant() { assert(offset <= data.length); }
this(in ubyte[] _data) { data = _data; }
void read(ref ubyte[] bytes) { bytes[] = data[offset..offset + bytes.length]; offset += bytes.length; }
void read(ref wchar[] chars) { read(cast(ubyte[]) chars); }
void read(ref dchar[] chars) { read(cast(ubyte[]) chars); }
void read(ref ubyte b) { b = data[offset]; offset += ubyte.sizeof; }
void read(ref byte b) { read(cast(ubyte)b); }
void read(ref char c) { read(cast(ubyte)c); }
void read(ref dchar c) { read(cast(uint)c); }
void read(ref ushort w) { w = *cast(ushort *)&data[offset]; offset += ushort.sizeof; }
void read(ref short s) { read(cast(ushort)s); }
void read(ref wchar c) { c = *cast(wchar *)&data[offset]; offset += wchar.sizeof; }
void read(ref uint w) { w = *cast(uint *)&data[offset]; offset += uint.sizeof; }
void read(ref int i) { read(cast(uint)i); }
void read(ref ulong l) { l = *cast(ulong *)&data[offset]; offset += ulong.sizeof; }
void read(ref long l) { read(cast(ulong)l); }
void read(ref float f) { f = *cast(float *)&data[offset]; offset += float.sizeof; }
void read(ref double f) { f = *cast(double *)&data[offset]; offset += double.sizeof; }
void read(ref real f) { f = *cast(real *)&data[offset] = f; offset += real.sizeof; }
void read(ref char[] s) { read(cast(ubyte[])s); }
}