add declare-and-bind back to Substance syntax

[kai-qu]

Is your feature request related to a problem? Please describe.
Having the ability to write T x := e instead of T x \n x := e would make life a lot easier for reading and writing Substance. (see e.g. pythagorean-theorem-sugared.sub). Also, as currently implemented in Haskell, I believe it is done specially in a parse stage, and is not even part of the syntactic sugar mechanism (since it should be available to all domains). Shouldn't be too much work.

I would like this to be available to users in the upcoming web port since we don't have a clear timeline for the syntactic sugar.

Describe the solution you'd like
As above

Describe alternatives you've considered
None

(Unless this is already implemented and is a bug? When I try to use it, the parser says

Unhandled Rejection (Error): Syntax error at line 3 col 14:

  Triangle ABC :=
               ^
Unexpected def token: ":=". Instead, I was expecting to see one of the following:

Can give more info if needed)

[wodeni]

Just added decl_bind to the parser in c6760e0. Closing now.

[kai-qu]

Thanks!!