Best interface for memcpy() (and the string.h family of functions)

[Kagamin]

IME partial copy primitives are lacking, so I use this:

/// Copy only as much as possible, return the copied data
T[] CopyHead(T)(T[] dst, in T[] src) pure
{
	if(dst.length>=src.length)return CopyAll(dst, src);
	CopyOverlap(dst, src[0..dst.length]);
	return dst;
}

/// Copy all input data, return the copied data
T[] CopyAll(T)(T[] dst, in T[] src) pure
{
	assert(dst.length>=src.length);
	dst=dst[0..src.length];
	CopyOverlap(dst, src);
	return dst;
}

/// Copy overlapping slices
void CopyOverlap(T)(T[] dst, in T[] src) pure
{
	import core.stdc.string:memmove;
	assert(dst.length==src.length,"same lengths required");
	byte[] dstBytes=cast(byte[])dst;
	memmove(dstBytes.ptr, src.ptr, dstBytes.length);
}