Use offer syntax for choice inside CP quotations.
Replaced typed_id with binder in the CP syntax.

The CP syntax used 'typed_id' in a number of places which would
eventually have to be desugared into Links binders.  Now, we use
'binder' in those places, instead of inventing a new (but equivalent)
type.
Add fuse.
Merge branch 'sessions' of github.com:slindley/links into sessions

Conflicts:
	desugarCP.ml
	sugartypes.ml
	transformSugar.ml
	typeSugar.ml
two factor authentication examples
Merge branch 'sessions'

Conflicts:
	desugarCP.ml
	examples/sessions/bookshop-cp.links
	examples/sessions/calc-cp.links
	examples/sessions/calc.links
	parser.mly
	sugartypes.ml
	transformSugar.ml
	typeSugar.ml
Start adding new forms, some frontend support.  Compiles.