come up with DAML-LF 2, which is exactly like DAML-LF 1.x, but without recursive data type

we need to get rid of the recursive data type in the DAML-LF proto. it causes endless trouble (see #437 for the latest in a long series of complaints). i propose to just have an array of exprs at the top level for each module, i.e.

message ExprTable {
  repeated Expr exprs = 1;
}

and then have pointers into said array instead of recursive occurrences.

however, we might as well clean up other stuff since we can:

• remove recursive data type
• remove party literals, getParty and the flag indicating the absence of party literals
• remove deprecated arrow type
• clean up identifier data type to be in line with what we have in Scala
• remove the variant of exercise which expects actors
• remove foldl, foldr and equal_list

@martin-drhu-da @remyhaemmerle-da please add items as you see fit.

[remyhaemmerle-da]

This has been way too long opened. cc @cocreature

[martin-drhu-da]

The flag indicating whether an expression contains party literals will be useless when there are no more party literals. And getParty seems also useless given there is from_text_party.