Dhall for Java 0.6.0-M1

This release updates the Dhall language version from 17.1.0 to 18.0.0.

As part of this update, we've added a With constructor to the abstract syntax tree for Dhall expressions, which allows us to implement the optimization discussed here. For example, the following expression previously took several seconds to parse in previous releases (and had a desugared string representation that was millions of characters long):

{a = {=}}
  with a.b = {=}
  with a.b.c = {=}
  with a.b.c.d = {=}
  with a.b.c.d.e = {=}
  with a.b.c.d.e.f = {=}
  with a.b.c.d.e.f.g = {=}
  with a.b.c.d.e.f.g.h = {=}
  with a.b.c.d.e.f.g.h.i = {=}
  with a.b.c.d.e.f.g.h.i.j = {=}

It now parses, normalizes, etc. instantaneously.

There is one known issue in this release, which was also a known issue in 0.5.0-M1 (see the release notes for details), but it only affects the dhall-imports Scala module, and should not affect most usage.