Refactor LurkField with lurk_ff changes

[johnchandlerburnham]

This refactor supersedes the lurk_ff crate from #191.

The essential pieces of this are:

• move tagging enums to src/tag.rs, since src.store.rs has way too much stuff in it.
• Improving various conversion functions for tags and fields
• adding some utilities which will be needed for serialization later
• Rename Tag -> ExprTag. This is needed because subsequent changes will require defining Tag as

enum Tag {
 Expr(ExprTag),
 Cont(ContTag),
 Op1(Op1),
 Op2(Op2),
}

which in turn allows for refactoring Ptr/ContPtr/ScalarPtr/ScalarContPtr.

This PR includes #237, which should be merged first.