refactor decl grammar to be more Rust like

[nikomatsakis]

I've been pondering the "decl layer". I think it might be a bit more readable if we made efforts to make it a touch more Rust like.

I thought we could collect ideas here in this tracking issue.

• where keyword in rust decl layer #45
• dedicated struct/union/enum forms in rust decl layer #46
• embed item names in the declarations
• make Self type parameter implicit on traits
• scalar types should be nameable #21

[nikomatsakis]

Some examples that came up in discussion today:

To represent static S: Foo = f-body, instead of

(StaticDecl (term (S (static () () (rigid-ty Foo ()) fn-body))))

we could do

(StaticDecl (term (static S () : (rigid-ty Foo ()) = fn-body where ())))

Note that this syntax supports generic statics/consts with where-clauses :)