Fixes #7

This is a breaking change.

The array element type now must be isbitstype:
Before, the chain of structs just had to be immutable. However, as seen in #7, the exact memory layout of non isbitstype structs is currently not well documented.

Switch from pointer to the strided array interface:
The pointer function is not recommended, and has subtle gotchas, especially if Julia is run with a moving GC.
The strided array interface is being used as documented in JuliaLang/julia#59466

The aref(a)[1]._3 syntax for accessing a field index is replaced with aref(a)[1].:3 or getproperty(aref(a)[1], 3).

This simplifies the code and avoids ambiguities when a field might be named _3.