Changed lambda literal syntax to be more concise (issue #1391)

[jemc]

This PR implements issue #1391 (RFC 24), changing the lambda syntax to be more concise, and more consistent with the "lambdatype" syntax, using curly braces as the bounds of the lambda.

Note that the implementation takes special care to give a convenient, comprehensible error message for the old lambda syntax, to try to make migration to the new syntax easier for people with existing codebases using the old syntax:

lambda ... end is no longer supported syntax; use {...} for lambdas
      lambda(s: String): String => s + " world" end
      ^

[Praetonus]

Shouldn't the old syntax still be part of the pony.g file? It seems to me that it always matches the output of ponyc --antlr.

[jemc]

@Praetonus I was under the impression that invalid syntax that gets weeded out at the syntax pass should not be included in the pony.g grammar. However, you're right, it matches the output of ponyc --antlr exactly.

So I added another commit where I regenerated pony.g with the following command:

ponyc --antlr > pony.g