Add _"squash"_ expressions to TatSu grammars

[apalala]

The idea is taken from peg in which the expression:

 < e1 e2 ... en >

yields the concatenation e1+e2+...+en.

[apalala]

There needs to be syntax for this, and it might be the +() grouping operator, as the + appears in TatSu grammars only in {}+.

rule == e1 +(e2 e3) e4`

Could yield: (e1, e2+e3, e4)

[apalala]

What happens is the types of the elements are different? The types can currently be: str, tuple, closure, AST, and Node. Define an algebra over the + operator?