Remove the EncodePtr trait

[SSheldon]

The EncodePtr trait may not be necessary. Currently, its main purpose is to express that reference/pointers to any NSObject may be encoded. This can partially be described using where clauses, like where *mut Self: Encode.

Unfortunately, this doesn't allow expressing where &'a Self: Encode because the lifetime is unconstrained, but working around this by casting to a pointer might not be so bad.

The main issue with this is rust-lang/rust#20671; even if a where clause is added to the INSObject trait, it doesn't propagate to subclasses.

[SSheldon]

This trait was removed in c50b646 after verifying messages was removed. Message verification can't be added back until the issue in rust is resolved.

[SSheldon]

Encode for references can be expressed with where for<'a> &'a Self: Encode