What: Support ArraySegment for underlying array

Why: It's useful to be able to read an SBE message from a view (i.e. ArraySegment) on a big array of bytes, without copies. Currently, we'd have to slice the byte array, which results in a copy.

How: similar to the byte[] case, pin the ArraySegment's underlying buffer so the GC doesn't move it, then offset the created pointer by the ArraySegment offset so that we point to the right memory.

Testing: run unit tests locally with ./runtests

Note: The C# wrappers should really be re-written using structs and Spans, which would enable fast and memory efficient code while still being safe, which the current code is not. Then again, that's a non-trivial undertaking, so this will suffice for now :)