Remove cyclic dependencies from lowlevel (don't use high level types)

[neilcsmith-net]

The current type mappers are problematic. Having lowlevel mappings using the highlevel types is inefficient. Having typed handlers / pointers in lowlevel would be more efficient, and also allow a future separation of lowlevel into a separate (auto-generated) library.

This can wait until post-1.0 if it can be proven we can do this without breaking API.

[neilcsmith-net]

Typed handler support added in for use in v1, and used in support for NativeObject.Handle, eg. for #123

Rest of work can bow happen post v1.