Casting a pointer and length value as a dynamic array

[solidstate1991]

I need this to port a C++ code to D (a compression algorithm 
known as LZHAM), and the easiest way to deal with it would be 
that. The ADLER32 and CRC32 algorithms had to be ditched, and 
while I could rewrite the former to make sense (used some form of 
"vectorization") I would like to use the standard library's 
implementation for the latter.