Replace dynamic trait objects with static type parameters

[mcevoypeter]

This PR replaces dynamic trait objects with static type parameters to eliminate the overhead of a vtable lookup at runtime. It does by requiring types implementing the NounAllocator or Stack traits also implement the Sized trait. This change does not result in an increase in binary size:

$ git checkout status
$ cargo build --release
$ du -sh target/release/libares.rlib
1.1M    target/release/libares.rlib

$ git checkout peter/remove-dyn
$ cargo build --release
$ du -sh target/release/libares.rlib
1.1M    target/release/libares.rlib

[eamsden]

Aside from separating formatting/semantic changes (de minimis in this case) this looks good.

[eamsden]

@mcevoypeter go ahead and merge this