Add generic ptr.To, ptr.Deref, ptr.Equal

This simplifies the implementation and adds support for pointers to
enums and arbitrary types.

To allow a complete migration from pointer to ptr, this also moves
AllPtrFieldsNil. The ptr.AllPtrFieldsNil "ptr" repetition is preserved
since the function only looks at pointer fields.

Existing deprecation notices are updated to point to the new
functions, but the remaining pointer functions aren't deprecated (yet)
to avoid having a large amount of imposed changes in downstream
projects where linting forbids deprecated functions. Existing test
code is preserved as-is to "prove" the correctness of the migration.

This was mostly written by Aldo Culquicondor; see
#269 for context.

Signed-off-by: Stephen Kitt <[email protected]>

Author: skitt