Make proptools functions consistently take *struct types

The proptools functions took an inconsistent variety of
struct and *struct types.  Some methods even took a struct
but returned a *struct.  Make all the exported methods
take a *struct, with internal helpers for the ones that need
to take a struct.

Test: proptools tests
Change-Id: I60ce212606e96adcef66c531d57f69c39e1a1638