Change Sugartypes.datatype to be a polymorphic variant type.
Simplify sugar datatype constructor names.
Merge session types into datatypes in the frontend.
Add state example. Fix linfork bugs.
Merge session types with datatypes in the backend. Now we can do type
aliases for session types.
Add bookshop examples.
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
Change "send" to "Send" to free up a keyword for session typing.
Rule out (most) form giving rise to Mix.

Access points, spawn, and close still provide unfettered access to mix,
of course.
Fix conflicts in the parser and merge session type parsing back into
datatype parsing.
Merge branch 'sessions' of github.com:slindley/links into sessions
Change names to more standard session typing names. Hack support for
EndBang in link.
Fix an outstanding bug with link... semi-colons are dangerous!
Irrelevant changes to typing error messages.
knife and reproduce
tweaks to calc examples