migrate to naga 0.13

[robtfm]

fixes #33

this should not merged until bevy 0.11.1 is out. builds on @VitalyAnkh's partial migration, with additionally :

• use Rc<RefCell> for const_expressions and const_expr_map - this is basically unavoidable, as DerivedModule::import_expression needs access to both the const_expressions and the arena to import into, which may also be const_expressions
• don't emit Expression::Literal and Expression::ZeroValues
• enforce const_expression uniqueness with a custom impl of PartialEq for Expressions, to ensure that the uniqueness test for globals and consts still passes (else we end up with duplicated items, they no longer test as equal as they refer to different init expressions). this could be removed if the PartialEq derive on Expression in naga is made externally available.
• a basic port of the prune module which just leaves all const_expressions present

the tests pass, but i haven't tried integrating into bevy (which will require bevy to use wgpu 0.17).

[Elabajaba]

It seems to work fine with bevy (the wasm issues are something else) bevyengine/bevy#9302

[JMS55]

Bevy 0.11.1 is out :)