Alignment and 2 GiB overflow issues in Span<T>.TryCopyTo

[mgravell]

Only affects the IsPrimitiveValueType<T> case;

• overflow for large src.Length and non-trivial T; might need to loop, since cpblk is limited to uint sizes (thus is not quite the same issue as Extensive 2 GiB overflow problems in Span<T> #846)
• CopyBlock assumes that the pointers are aligned; otherwise, cpblk needs the unaligned prefix

(also impacts ReadOnlySpan<T>)

[benaadams]

Raised issue for unaligned https://github.com/dotnet/corefx/issues/12243